Logic circuit

ABSTRACT

An object is to apply a transistor using an oxide semiconductor to a logic circuit including an enhancement transistor. The logic circuit includes a depletion transistor  101  and an enhancement transistor  102 . The transistors  101  and  102  each include a gate electrode, a gate insulating layer, a first oxide semiconductor layer, a second oxide semiconductor layer, a source electrode, and a drain electrode. The transistor  102  includes a reduction prevention layer provided over a region in the first oxide semiconductor layer between the source electrode and the drain electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit including a thin filmtransistor using an oxide semiconductor. Specifically, the presentinvention relates to a logic circuit.

2. Description of the Related Art

A thin film transistor (TFT) formed over a flat plate such as a glasssubstrate, which is typically used in a liquid crystal display device,is generally formed using a semiconductor material such as amorphoussilicon or polycrystalline silicon. TFTs using amorphous silicon have alow electric field mobility but can respond to increase in size of glasssubstrates. On the other hand, TFTs using polycrystalline silicon have ahigh electric field mobility, but need a crystallization step such aslaser annealing and are not always adaptable to increase in size ofglass substrates.

Thus, a technique in which a TFT is formed using an oxide semiconductoras a semiconductor material and applied to an electronic device or anoptical device has attracted attention. For example, Patent Documents 1and 2 each disclose a technique in which a TFT is formed using zincoxide or an In—Ga—Zn—O-based oxide semiconductor as a semiconductormaterial and used for a switching element or the like in an imagedisplay device.

A TFT in which a channel formation region (also referred to as a channelregion) is provided in an oxide semiconductor can have a higher electricfield mobility than a TFT using amorphous silicon. An oxidesemiconductor film can be formed at a temperature of 300° C. or lower bya sputtering method or the like, and a manufacturing process of the TFTusing an oxide semiconductor is simpler than that of the TFT usingpolycrystalline silicon.

TFTs which are formed using such an oxide semiconductor over a glasssubstrate, a plastic substrate, or the like are expected to be appliedto display devices such as a liquid crystal display, anelectroluminescent display (also referred to as an EL display), andelectronic paper.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2007-123861-   Patent Document 2: Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

However, a conventional TFT using an oxide semiconductor tends to be adepletion-type TFT, which is normally on, and the threshold voltage ofthe TFT shifts over time. Accordingly, it has been difficult to applythe conventional TFT using the oxide semiconductor to a logic circuitconstituted by a transistor having a desired threshold voltage, forexample, an enhancement-type transistor, which is normally off.

In view of the forgoing problems, an object is to obtain a desiredthreshold voltage of a thin film transistor using an oxidesemiconductor, and specifically, an object is to apply the thin filmtransistor to a logic circuit constituted by a transistor having adesired threshold voltage.

One embodiment of the invention disclosed in this specification is alogic circuit including an enhancement transistor which includes a layerfor preventing reduction over a back channel so that the thresholdvoltage is controlled.

One embodiment is a specific structure of a logic circuit described asfollows. The logic circuit includes a depletion transistor in which ahigh power supply voltage is applied to one of a source and a drain, anda gate is electrically connected to the other of the source and thedrain; and an enhancement transistor in which a first signal is input toa gate, one of a source and a drain is electrically connected to theother of the source and the drain of the depletion transistor, and a lowpower supply voltage is applied to the other of the source and thedrain. The enhancement transistor outputs as a second signal a voltageof a portion where the enhancement transistor is connected to thedepletion transistor. Each of the depletion transistor and theenhancement transistor includes a gate electrode; a gate insulatinglayer provided over the gate electrode; a first oxide semiconductorlayer provided over the gate insulating layer; a pair of second oxidesemiconductor layers in contact with part of the first oxidesemiconductor layer, serving as a source region and a drain region; asource electrode in contact with one of the second oxide semiconductorlayers, which is the source region; and a drain electrode in contactwith the other of the second oxide semiconductor layers, which is thedrain region. The enhancement transistor includes a reduction preventionlayer over a region in the first oxide semiconductor layer between thesource electrode and the drain electrode.

One embodiment is another specific structure of a logic circuitdescribed as follows. The logic circuit includes first transistor inwhich a first clock signal is input to a gate, and an input signal isinput to the one of a source and a drain; a first inverter whose inputterminal is electrically connected to the other of the source and thedrain of the first transistor; a second inverter whose input terminal iselectrically connected to an output terminal of the first inverter; athird inverter having an input terminal electrically connected to theoutput terminal of the first inverter, and an output terminal outputtingan output signal; and a second transistor in which a second clock signalis input to a gate, one of a source and a drain is electricallyconnected to the other of the source and the drain of the firsttransistor, and the other of the source and the drain is electricallyconnected to an output terminal of the second inverter. Each of thefirst inverter and the second inverter includes a depletion transistorin which a high power supply voltage is applied to one of a source and adrain, and a gate is electrically connected to the other of the sourceand the drain; and an enhancement transistor in which a first signal isinput to a gate, one of a source and a drain is electrically connectedto the other of the source and the drain of the depletion transistor,and a low power supply voltage is applied to the other of the source andthe drain. The enhancement transistor outputs as a second signal avoltage of a portion where the enhancement transistor is connected tothe depletion transistor. Each of the depletion transistor and theenhancement transistor includes a gate electrode; a gate insulatinglayer provided over the gate electrode; a first oxide semiconductorlayer provided over the gate insulating layer; a pair of second oxidesemiconductor layers in contact with part of the first oxidesemiconductor layer, serving as a source region and a drain region; asource electrode in contact with one of the second oxide semiconductorlayers, which is the source region; and a drain electrode in contactwith the other of the second oxide semiconductor layers, which is thedrain region. The enhancement transistor includes a reduction preventionlayer over a region in the first oxide semiconductor layer between thesource electrode and the drain electrode.

The enhancement transistor can include an oxygen vacancy control regionbetween the source electrode and the drain electrode over a surface ofthe first oxide semiconductor layer, which is opposite to a surface incontact with the gate insulating layer.

Each of the first oxide semiconductor layer and the second oxidesemiconductor layers can contain indium, gallium, and zinc.

One embodiment is another specific structure of a logic circuitdescribed as follows. The logic circuit includes a depletion transistorin which a high power supply voltage is applied to one of a source and adrain, and a gate is electrically connected to the other of the sourceand the drain; and an enhancement transistor in which a first signal isinput to a gate, one of a source and a drain is electrically connectedto the other of the source and the drain of the depletion transistor,and a low power supply voltage is applied to the other of the source andthe drain. The enhancement transistor outputs as a second signal avoltage of a portion where the enhancement transistor is connected tothe depletion transistor. Each of the depletion transistor and theenhancement transistor includes a gate electrode; a gate insulatinglayer provided over the gate electrode; an oxide semiconductor layerprovided over the gate insulating layer; and a source electrode and adrain electrode in contact with part of the oxide semiconductor layer.The enhancement transistor includes a reduction prevention layer over aregion in the oxide semiconductor layer between the source electrode andthe drain electrode.

One embodiment is another specific structure of a logic circuitdescribed as follows. The logic circuit includes a first transistor inwhich a first clock signal is input to a gate, and an input signal isinput to the one of a source and a drain; a first inverter whose inputterminal is electrically connected to the other of the source and thedrain of the first transistor; a second inverter whose input terminal iselectrically connected to the output terminal of the first inverter; athird inverter having an input terminal electrically connected to theoutput terminal of the first inverter, and an output terminal outputtingan output signal; and a second transistor in which a second clock signalis input to a gate, one of a source and a drain is electricallyconnected to the other of the source and the drain of the firsttransistor, and the other of the source and the drain is electricallyconnected to the output terminal of the second inverter. Each of thefirst inverter and the second inverter includes a depletion transistorin which a high power supply voltage is applied to one of a source and adrain, and a gate is electrically connected to the other of the sourceand the drain; and an enhancement transistor in which a first signal isinput to a gate, one of a source and a drain is electrically connectedto the other of the source and the drain of the depletion transistor,and a low power supply voltage is applied to the other of the source andthe drain. The enhancement transistor outputs as a second signal avoltage of a portion where the enhancement transistor is connected tothe depletion transistor. Each of the depletion transistor and theenhancement transistor includes a gate electrode; a gate insulatinglayer provided over the gate electrode; an oxide semiconductor layerprovided over the gate insulating layer; and a source electrode and adrain electrode in contact with part of the oxide semiconductor layer.The enhancement transistor includes a reduction prevention layer over aregion in the oxide semiconductor layer between the source electrode andthe drain electrode.

The enhancement transistor can include an oxygen vacancy control regionbetween the source electrode and the drain electrode over a surface ofthe first oxide semiconductor layer, which is opposite to a surface incontact with the gate insulating layer.

The oxide semiconductor layer can contain indium, gallium, and zinc.

The depletion transistor and the enhancement transistor can have thesame conductivity type.

The source electrode or the drain electrode of the depletion transistorcan be in contact with the gate electrode of the enhancement transistorthrough an opening portion provided in the gate insulating layer.

An oxide semiconductor used in this specification is represented byInMO₃(ZnO), (m>0). Note that M represents one or more of metal elementsselected from gallium (Ga), iron (Fe), nickel (Ni), manganese (Mn), orcobalt (Co). As an example, M may be Ga or may include the above metalelement in addition to Ga, for example, M may be Ga and Ni or Ga and Fe.Moreover, the oxide semiconductor may contain a transition metal elementsuch as Fe or Ni or oxide of the transition metal element as an impurityelement in addition to the metal element contained as M. Note that inthis specification, an oxide semiconductor film containing indium,gallium, and zinc is also referred to as an In—Ga—Zn—O-basednon-single-crystal film.

Since the In—Ga—Zn—O-based non-single-crystal film is formed by asputtering method and subjected to heat treatment at a temperature of200° C. to 500° C., specifically 300° C. to 400° C. for 10 to 100minutes, the amorphous structure is observed by X-ray diffraction (XRD)analysis as a crystal structure. Moreover, as for electriccharacteristics, a TFT with an on/off ratio of 10⁹ or more and amobility of 10 or more in the case where the gate voltage is ±20 V canbe manufactured.

Note that in this document (the specification, the scope of claims, thedrawings, and the like), a logic circuit performs logic operation basedon a signal input thereto and outputs a signal in accordance with theresult of the operation. For example, the logic circuit includes acombinational logic circuit (e.g., a NOT circuit and a NAND circuit) anda sequential logic circuit (e.g., a flip flop circuit and a shiftregister) in its category.

An enhancement thin film transistor using an oxide semiconductor, inwhich the shift of the threshold voltage over time is suppressed, can beprovided, whereby the transistor using the oxide semiconductor can beapplied to a logic circuit including an enhancement transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram illustrating a circuit configuration of alogic circuit in Embodiment 1;

FIGS. 2A and 2B are circuit diagrams for illustrating operation of alogic circuit in Embodiment 1;

FIG. 3 is a circuit diagram illustrating a circuit configuration of alogic circuit in Embodiment 1;

FIGS. 4A and 4B are circuit diagrams for illustrating operation of alogic circuit in Embodiment 1;

FIGS. 5C and 5D are circuit diagrams for illustrating operation of alogic circuit in Embodiment 1;

FIG. 6 is a timing chart illustrating operation of a logic circuit inEmbodiment 1;

FIG. 7 is a circuit diagram illustrating a circuit configuration of alogic circuit in Embodiment 1;

FIGS. 8A and 8B are circuit diagrams for illustrating operation of alogic circuit in Embodiment 1;

FIGS. 9A to 9C each illustrate a structure of a logic circuit inEmbodiment 1;

FIGS. 10A and 10B illustrate a structure of a logic circuit inEmbodiment 1;

FIG. 11 is a circuit diagram illustrating a circuit configuration of alogic circuit in Embodiment 2;

FIG. 12 is a circuit diagram illustrating a circuit configuration of aNAND circuit in Embodiment 2;

FIGS. 13A and 13B are circuit diagrams each illustrating operation of aNAND circuit in Embodiment 2;

FIG. 14 is a timing chart illustrating operation of a logic circuit inEmbodiment 2;

FIGS. 15A and 15B illustrate a structure of a logic circuit inEmbodiment 3;

FIGS. 16A and 16B illustrate a structure of a logic circuit inEmbodiment 4;

FIGS. 17A and 17B are cross-sectional views illustrating a method formanufacturing a logic circuit in Embodiment 5;

FIGS. 18C and 18D are cross-sectional views illustrating a method formanufacturing a logic circuit in Embodiment 5;

FIG. 19 is a block diagram illustrating a structure of a display devicein Embodiment 6;

FIGS. 20A and 20B are block diagrams each illustrating a structure of adriver circuit in a display device shown in Embodiment 6;

FIG. 21 is a circuit diagram illustrating a circuit configuration of apixel in a display device in Embodiment 7;

FIGS. 22A and 22B illustrate a structure of a pixel in a display devicein Embodiment 7;

FIGS. 23A to 23D each illustrate a structure of a pixel in a displaydevice in Embodiment 7;

FIG. 24 is a circuit diagram illustrating a circuit configuration of apixel in a display device in Embodiment 8;

FIGS. 25A to 25C are cross-sectional views each illustrating a structureof a pixel in a display device in Embodiment 8;

FIGS. 26A and 26B illustrate a structure of a display device inEmbodiment 8;

FIG. 27 is a cross-sectional view illustrating a structure of electronicpaper in Embodiment 9;

FIG. 28 illustrates an electronic device to which electronic paper inEmbodiment 9 is applied;

FIGS. 29A to 29C each illustrate a structure of a display device inEmbodiment 10;

FIGS. 30A and 30B each illustrate an electronic device in Embodiment 11;

FIGS. 31A and 31B each illustrate an electronic device in Embodiment 11;

FIGS. 32A and 32B each illustrate an electronic device in Embodiment 11;and

FIGS. 33A and 33B each illustrate a thin film transistor in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to the accompanyingdrawings. Note that the invention disclosed in this specification is notlimited to the following description, and it is easily understood bythose skilled in the art that modes and details can be variously changedwithout departing from the spirit and the scope of the invention.Therefore, the invention disclosed in this specification is notconstrued as being limited to the description of the followingembodiments.

Embodiment 1

In this embodiment, one embodiment of a logic circuit will be described.

First, a circuit configuration of a logic circuit in this embodimentwill be described with reference to FIG. 1. FIG. 1 is a circuit diagramillustrating a circuit configuration of a logic circuit in thisembodiment.

The logic circuit illustrated in FIG. 1 is a combinational logic circuitincluding a transistor 101 and a transistor 102.

Note that in this document (the specification, the scope of claims, thedrawings, and the like), a transistor has at least three terminals of agate, a source, and a drain.

The gate is the entire gate electrode and gate wiring or part thereof.The gate wiring is a wiring for electrically connecting a gate electrodeof at least one transistor to another electrode or another wiring, andincludes a scan line in a display device in its category, for example.

The source is the entire source region, source electrode, and sourcewiring or part thereof. The source region indicates a region in asemiconductor layer, where the resistivity is equal to or less than agiven value. The source electrode indicates part of a conductive layer,which is connected to the source region. The source wiring is a wiringfor electrically connecting a source electrode of at least onetransistor to another electrode or another wiring. For example, in thecase where a signal line in a display device is electrically connectedto a source electrode, the source wiring includes the signal line in itscategory.

The drain is the entire drain region, drain electrode, and drain wiringor part thereof. The drain region indicates a region in a semiconductorlayer, where the resistivity is equal to or less than a given value. Thedrain electrode indicates part of a conductive layer, which is connectedto the drain region. The drain wiring is a wiring for electricallyconnecting a drain electrode of at least one transistor to anotherelectrode or another wiring. For example, in the case where a signalline in a display device is electrically connected to a drain electrode,the drain wiring includes the signal line in its category.

In addition, in this document (the specification, the scope of claims,the drawings, and the like), a source and a drain of a transistor changedepending on the structure, the operating conditions, or the like of thetransistor; therefore, it is difficult to determine which is the sourceand which is the drain. Accordingly, in this document (thespecification, the scope of claims, the drawings, and the like), oneterminal which is freely selected from the source and the drain isreferred to as one of the source and the drain, whereas the otherterminal is referred to as the other of the source and the drain.

The transistor 101 is a depletion-type transistor (also referred to as adepletion transistor). One of a source and a drain of the transistor 101is electrically connected to a power supply line 103, and a high powersupply voltage (Vdd) is applied to one of the source and the drainthrough the power supply line 103. Moreover, a gate and the other of thesource and the drain of the transistor 101 are electrically connected toeach other (i.e., the transistor 101 is diode-connected). Note that anexample of a depletion transistor is a transistor whose thresholdvoltage is negative in the case of an n-channel transistor.

Note that in general, a voltage refers to the difference betweenpotentials of two points (also referred to as the potential difference),and a potential refers to electrostatic energy (electric potentialenergy) that a unit charge in an electrostatic field at one point has.However, in an electric circuit, the potential difference between apotential at one point and a potential serving as a reference (alsoreferred to as a reference potential) is sometimes used as a value, forexample. Moreover, both the value of a voltage and the value of apotential are represented by volts (V); therefore, in the document (thespecification and the scope of claims) of the present application, avoltage at one point is sometimes used as a value unless otherwisespecified.

The transistor 102 is an enhancement-type transistor (also referred toas an enhancement transistor). One of a source and a drain of thetransistor 102 is electrically connected to the other of the source andthe drain of the transistor 101. The other of the source and the drainof the transistor 102 is electrically connected to a power supply line104, and a low power supply voltage (Vss) is applied to the other of thesource and the drain of the transistor 102 through the power supply line104. The low power supply voltage is a ground potential (VGND) or agiven voltage, for example. Note that an example of an enhancementtransistor is a transistor whose threshold voltage is positive in thecase of an n-channel transistor.

The high power supply voltage is relatively higher than the low powersupply voltage, and the low power supply voltage is relatively lowerthan the high power supply voltage. Each value is set as appropriatebased on specifications of a circuit or the like, and thus there is noparticular limitation on the value. For example, when Vdd>Vss, Vdd>Vssis not always satisfied. Moreover, when Vdd>Vss, VGND≧Vss is not alwayssatisfied.

Further, transistors of the same conductivity type can be used for thetransistors 101 and 102. In this embodiment, the case where thetransistors 101 and 102 are n-channel transistors is described as anexample.

Next, operation of the logic circuit illustrated in FIG. 1 will bedescribed. In the logic circuit in this embodiment, a first signal isinput to a gate of the transistor 102, and a voltage at a portion (alsoreferred to as a node) 105 where the transistors 101 and 102 areconnected to each other is output as a second signal. Specific operationof the logic circuit will be described below.

The operation of the logic circuit in this embodiment can be classifiedinto two categories depending on whether the first signal is in a lowstate or a high state. A low state is a state where a voltage isrelatively low as compared to a high state, and a high state is a statewhere a voltage is relatively high as compared to a low state. Bothcases will be described with reference to FIGS. 2A and 2B. FIGS. 2A and2B illustrate operation of the logic circuit in this embodiment. Notethat in this embodiment, the case where data is 0 in a low state anddata is 1 in a high state is described as an example; however, oneembodiment of the invention is not limited thereto, and data can be 1 ina low state and can be 0 in a high state. Note that a voltage in a lowstate is referred to a low voltage (VL), and a voltage in a high stateis referred to a high voltage (VH). Values of the low voltage and thehigh voltage are not limited to specific values, and the low voltageshould be equal to or lower than a given value and the high voltageshould be equal to or higher than the given value.

FIG. 2A illustrates the operation in the case where a voltage (V1) ofthe first signal is high (i.e., V1=VH). As illustrated in FIG. 2A, inthe case where V1=VH, the transistor 102 is turned on. When thetransistor 102 is on, the resistance (R102) of the transistor 102 islower than the resistance (R101) of the transistor 101 (i.e.,R102<R101); accordingly, a voltage (V105) of the node 105 is VL and avoltage (V2) of the second signal is VL.

FIG. 2B illustrates the operation in the case where V1=VL. Asillustrated in FIG. 2B, in the case where V1=VL, the transistor 102 isturned off. When the transistor 102 is off, R102 is higher than R101, sothat V105 is VH and V2 is VH. At this time, the value of VH, which isthe voltage of the second signal, is (Vdd−Vth101) (Vth101 represents thethreshold voltage of the transistor 101). The above is the operation ofthe logic circuit illustrated in FIG. 1.

Further, a sequential logic circuit can be constituted by thecombinational logic circuit illustrated in FIG. 1. A circuitconfiguration of a logic circuit using a combinational circuit will bedescribed with reference to FIG. 3. FIG. 3 is a circuit diagramillustrating a circuit configuration of a logic circuit in thisembodiment.

A logic circuit illustrated in FIG. 3 includes a transistor 111, aninverter 1121, an inverter 1122, an inverter 1123, and a transistor 113.

A first clock signal (CL1) is input to a gate of the transistor 111, anda signal is input to one of a source and a drain of the transistor 111.The signal input to one of the source and the drain is referred to as aninput signal.

An input terminal of the inverter 1121 is electrically connected to theother of the source and the drain of the transistor 111.

An input terminal of the inverter 1122 is electrically connected to anoutput terminal of the inverter 1121.

An input terminal of the inverter 1123 is electrically connected to theoutput terminal of the inverter 1121. A second signal is output from anoutput terminal of the inverter 1123.

The logic circuit illustrated in FIG. 1 can be applied to each of theinverters 1121 to 1123.

A second clock signal (CL2) is input to a gate of the transistor 113.One of a source and a drain of the transistor 113 is electricallyconnected to the other of the source and the drain of the transistor111. The other of the source and the drain of the transistor 113 iselectrically connected to an output terminal of the inverter 1122.

The first clock signal and the second clock signal each have two statesof a high state and a low state. A voltage in a high state is a highvoltage, and a voltage in a low state is a low voltage.

Moreover, the first clock signal and the second clock signal haveopposite phases. For example, in a predetermined period, the secondclock signal is low when the first clock signal is high, whereas thesecond clock signal is high when the first clock signal is low.

Note that in this embodiment, the case is described in which the firstclock signal is input to the gate of the transistor 111 and the secondclock signal is input to the gate of the transistor 113; however, oneembodiment of the invention is not limited thereto, and a structure canbe employed in which the second clock signal is input to the gate of thetransistor 111 and the first clock signal is input to the gate of thetransistor 113.

Next, operation of the logic circuit illustrated in FIG. 3 will bedescribed with reference to FIGS. 4A and 4B, FIGS. 5C and 5D, and FIG.6. FIGS. 4A and 4B and FIGS. 5C and 5D illustrate the operation of thelogic circuit in FIG. 3. FIG. 6 is a timing chart illustrating theoperation of the logic circuit in FIG. 3.

The operation of the logic circuit illustrated in FIG. 3 is mainlyclassified into four periods. Each period will be described below.

First, in a first period, as illustrated in FIG. 6, the first clocksignal is high, that is, CL1 is VH and the second clock signal is low,that is, CL2 is VL. Accordingly, the transistor 111 is turned on and thetransistor 113 is turned off as illustrated in FIG. 4A. Moreover, avoltage (Vin) of the input signal is a high voltage, that is, Vin is VH.

At this time, since the transistor 111 is on, a voltage (V114) of thenode 114 is VH. Since the voltage of the node 114 is applied to theinput terminal of the inverter 1121, a signal of VL is output from theinverter 1121, and a voltage (V115) of a node 115 is VL. Further, sincethe voltage of the node 115 is applied to the input terminal of theinverter 1122, a signal of VH is output from the inverter 1122. However,the voltage of the output signal from the inverter 1122 is not appliedto the node 114 because the transistor 113 is off. Moreover, the voltageof the node 115 is also applied to the input terminal of the inverter1123, so that a signal of VH is output from the inverter 1123 asillustrated in FIG. 4A. The above is the operation in the first period.

Next, in a second period, as illustrated in FIG. 6, CL1 is VL and CL2 isVH; accordingly, the transistor 111 is turned off and the transistor 113is turned on as illustrated in FIG. 4B. Moreover, Vin is VL.

At this time, since the transistor 111 is off, V114 remains VH even whenVin is VL. Since the voltage of the node 114 is applied to the inputterminal of the inverter 1121, a signal of VL is output from theinverter 1121, and V115 remains VL. Further, the voltage of the node 115is applied to the input terminal of the inverter 1122, and a signal ofVH is output from the inverter 1122. Moreover, since the transistor 113is off, the voltage of the signal from the inverter 1122 is applied tothe node 114. The voltage of the node 115 is also applied to the inputterminal of the inverter 1123, so that a signal of VH is output from theinverter 1123 as illustrated in FIG. 4B. The above is the operation inthe second period.

Next, in a third period, as illustrated in FIG. 6, CL1 is VH and CL2 isVL; accordingly, the transistor 111 is turned on and the transistor 113is turned off as illustrated in FIG. 5C. Moreover, Vin remains VL.

At this time, since the transistor 111 is on, V114 is VH. Since thevoltage of the node 114 is applied to the input terminal of the inverter1121, a signal of VH is output from the inverter 1121, and V115 is VH.Further, since the voltage of the node 115 is applied to the inputterminal of the inverter 1122, a signal of VL is output from theinverter 1122. However, the voltage of the output signal from theinverter 1122 is not applied to the node 114 because the transistor 113is off. Moreover, the voltage of the node 115 is also applied to theinput terminal of the inverter 1123, so that a signal of VL is outputfrom the inverter 1123 as illustrated in FIG. 5C. The above is theoperation in the third period.

Next, in a fourth period, as illustrated in FIG. 6, CL1 is VL and CL2 isVH; accordingly, the transistor 111 is turned off and the transistor 113is turned on as illustrated in FIG. 5D. Moreover, Vin remains VL.

At this time, since the transistor 111 is off, V114 remains VL. SinceV114 is VL, a signal of VH is output from the inverter 1121, and V115remains VH. Further, since V115 is VH, a signal of VL is output from theinverter 1122, and since the transistor 113 is on, the voltage of thesignal from the inverter 1122 is applied to the node 114. Moreover, thevoltage of the node 115 is also applied to the input terminal of theinverter 1123, so that a signal of VL is output from the inverter 1123as illustrated in FIG. 5D. The above is the operation in the fourthperiod.

Through the above operation, the logic circuit illustrated in FIG. 3 cangenerate an output signal based on a state of a signal input thereto.

Note that in the logic circuit illustrated in FIG. 3, a combinationallogic circuit using a bootstrap method can be applied to the inverter1123. A logic circuit using a bootstrap method will be described withreference to FIG. 7. FIG. 7 is a circuit diagram illustrating a circuitconfiguration of a logic circuit using a bootstrap method in thisembodiment.

The logic circuit illustrated in FIG. 7 includes a transistor 106, acapacitor 108, and a capacitor 109 in addition to the circuitconfiguration of the logic circuit illustrated in FIG. 1, and alsoincludes a transistor 107 instead of the transistor 101. In the logiccircuit in FIG. 7, the description of the logic circuit in FIG. 1 isemployed as appropriate for the same portion as the logic circuit inFIG. 1.

A gate and one of a source and a drain of the transistor 106 areelectrically connected to the power supply line 103, and the high powersupply voltage is applied to the gate and one of the source and thedrain. The other of the source and the drain of the transistor 106 iselectrically connected to a gate of the transistor 107.

The gate of the transistor 107 is electrically connected to the other ofthe source and the drain of the transistor 106. One of a source and adrain the transistor 107 is electrically connected to the power supplyline 103, and the high power supply voltage is applied to one of thesource and the drain.

The capacitor 108 has a first terminal and a second terminal. The firstterminal is electrically connected to the other of the source and thedrain of the transistor 106, and the second terminal is electricallyconnected to the other of the source and the drain of the transistor107.

The capacitor 109 has a first terminal and a second terminal. The firstterminal is electrically connected to the other of the source and thedrain of the transistor 107. The second terminal is electricallyconnected to the power supply line 104, and the low power supply voltageis applied to the second terminal.

Next, operation of the logic circuit illustrated in FIG. 7 will bedescribed.

In the logic circuit in FIG. 7, as in the logic circuit in FIG. 1, thefirst signal is input to the gate of the transistor 102, and a voltageof a node 1111 between the transistors 107 and 102 is output as thesecond signal.

The operation of the logic circuit illustrated in FIG. 7 can beclassified into two categories depending on whether the voltage of thefirst signal is low or high. Both cases will be described with referenceto FIGS. 8A and 8B. FIGS. 8A and 8B illustrate operation of the logiccircuit in this embodiment. Note that in this embodiment, the case wheredata is 0 in a low state and data is 1 in a high state is described asan example; however, one embodiment of the invention is not limitedthereto, and data can be 1 in a low state and can be 0 in a high state.

FIG. 8A illustrates the operation in the case where V1=VH. Asillustrated in FIG. 8A, in the case where V1=VH, the transistor 102 isturned on. When the transistor 102 is on, the resistance of thetransistor 102 is lower than the resistance (R107) of the transistor 107(i.e., R102<R107), and a voltage (V1111) of the node 1111 is VL; thus,V2 is VL. Further, the transistor 106 is turned off at the time when avoltage of a node 110 between the other of the source and the drain ofthe transistor 106 and the gate of the transistor 107 becomes a valueobtained by subtracting the threshold voltage (Vth106) of the transistor106 from the high power supply voltage, that is, (Vdd−Vth106), and thenode 110 enters into a floating state.

FIG. 8B illustrates the operation in the case where V1=VL. Asillustrated in FIG. 8B, in the case where V1=VL, the transistor 102 isturned off. When the transistor 102 is off, R102 is higher than R107,and the voltage of the node 1111 is increased by the capacitor 109 andthe voltage of the node 110 is also increased by capacitive couplingwith the capacitor 108. Thus, it follows that V2=V110=V1111=VH. At thistime, the value of VH is larger than VH, which is the voltage of thesecond signal in the logic circuit illustrated in FIG. 1, and expressedas VH=Vdd+Vth106. The above is the operation of the logic circuitillustrated in FIG. 7.

As described above, by using the logic circuit in FIG. 7 as the inverter1123, the voltage of the second signal can be amplified.

Next, a structure of the logic circuit in FIG. 1 will be described withreference to FIGS. 9A to 9C. FIGS. 9A to 9C each illustrate a structureof the logic circuit in FIG. 1. FIG. 9A is a top view. Each of FIGS. 9Band 9C is a cross-sectional view of the logic circuit along Z1-Z2 inFIG. 9A.

As illustrated in FIGS. 9A and 9B, the logic circuit in this embodimentincludes a transistor 201 and a transistor 202. Specifically, the logiccircuit includes a substrate 210; gate electrodes 2111 and 2112 over thesubstrate 210; a gate insulating layer 212 provided so as to cover thegate electrodes 2111 and 2112; an oxide semiconductor layer 2131provided over the gate insulating layer 212 over the gate electrode2111; an oxide semiconductor layer 2132 provided over the gateinsulating layer 212 over the gate electrode 2112; oxide semiconductorlayers 2141 a, 2141 b, 2142 a, and 2142 b; and a reduction preventionlayer 218.

The transistor 201 corresponds to the transistor 101 in FIG. 1. The gateelectrode 2111 is provided over the substrate 210. The gate insulatinglayer 212 is provided over the gate electrode 2111. The oxidesemiconductor layer 2131 is provided over the gate insulating layer 212.The oxide semiconductor layers 2141 a and 2141 b, which are a pair ofoxide semiconductor layers, are provided over the oxide semiconductorlayer 2131. Electrodes 215 and 216, which are a pair of electrodes, areprovided so as to be in contact with the oxide semiconductor layers 2141a and 2141 b, respectively.

When it is explicitly described that B is formed on or over A, it doesnot necessarily mean that B is formed in direct contact with A. Thedescription includes the case where A and B are not in direct contactwith each other, that is, the case where another object is placedbetween A and B. Here, each of A and B corresponds to an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, afilm, or a layer).

Therefore, for example, when it is explicitly described that a layer Bis formed on or over a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or the layer D. Note that another layer (e.g., the layer C orthe layer D) may be a single layer or a plurality of layers.

The transistor 202 corresponds to the transistor 102 in FIG. 1. The gateelectrode 2112 is provided over the substrate 210. The gate insulatinglayer 212 is provided over the gate electrode 2112. The oxidesemiconductor layer 2132 is provided over the gate insulating layer 212.The oxide semiconductor layers 2142 a and 2142 b, which are a pair ofoxide semiconductor layers, are provided over the oxide semiconductorlayer 2132. The electrode 216 and an electrode 217, which are a pair ofelectrodes, are provided so as to be in contact with the oxidesemiconductor layers 2142 a and 2142 b, respectively. The reductionprevention layer 218 is provided over the oxide semiconductor layer2132.

For the substrate 210, an alkali-free glass substrate manufactured by afusion method or a float method, such as a substrate of bariumborosilicate glass, aluminoborosilicate glass, or aluminosilicate glass;a ceramic substrate; a plastic substrate which has high heat resistanceenough to withstand a process temperature of this manufacturing process;or the like can be used. As the plastic substrate, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used, forexample. Moreover, a sheet in which aluminum foil is placed between PVFfilms or polyester films can be used as the substrate.

The gate electrodes 2111 and 2112 can be formed with a single-layerstructure or a layered structure using a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material containing any of thesematerials as a main component, for example. The edges of the gateelectrodes 2111 and 2112 are preferably tapered.

For example, for a two-layer structure of the gate electrodes 2111 and2112, it is preferable to employ any of the following two-layerstructures: a structure where a molybdenum layer is stacked over analuminum layer; a structure where a molybdenum layer is stacked over acopper layer; a structure where a titanium nitride layer or a tantalumnitride layer is stacked over a copper layer; a structure where atitanium nitride layer and a molybdenum layer are stacked. For a layeredstructure, a tungsten layer or a tungsten nitride layer, a layer of analloy of aluminum and silicon or an alloy of aluminum and titanium, anda titanium nitride layer or a titanium layer are preferably stacked.

For the gate insulating layer 212, one of oxide, nitride, oxynitride,and nitride oxide of silicon, aluminum, yttrium, tantalum, or hafnium;or a compound containing at least two such materials can be used.Moreover, a halogen element such as chlorine or fluorine may becontained in the gate insulating layer 212.

The oxide semiconductor layers 2131 and 2132 are first oxidesemiconductor layers. As the oxide semiconductor layers 2131 and 2132,an In—Ga—Zn—O-based non-single-crystal film can be used, for example.

The oxide semiconductor layers 2141 a, 2141 b, 2142 a, and 2142 b aresecond oxide semiconductor layers and function as source regions anddrain regions. The oxide semiconductor layers 2141 a, 2141 b, 2142 a,and 2142 b are formed using, for example, an In—Ga—Zn—O-basednon-single-crystal film formed under deposition conditions which aredifferent from those of the oxide semiconductor layers 2131 and 2132.For example, when the oxide semiconductor layers 2141 a, 2141 b, 2142 a,and 2142 b are formed using an oxide semiconductor film obtained under acondition where the flow rate of an argon gas for sputtering is 40 sccm,they have n-type conductivity and have an activation energy (ΔE) of 0.01eV to 0.1 eV. Note that in this embodiment, the oxide semiconductorlayers 2141 a, 2141 b, 2142 a, and 2142 b are In—Ga—Zn—O-basednon-single-crystal films and include at least amorphous components.Moreover, the oxide semiconductor layers 2141 a, 2141 b, 2142 a, and2142 b may include crystal grains (nanocrystals). The crystal grain (thenanocrystal) in the oxide semiconductor layers 2141 a, 2141 b, 2142 a,and 2142 b has a diameter of 1 nm to 10 nm, typically approximately 2 nmto 4 nm.

Note that the oxide semiconductor layers 2141 a, 2141 b, 2142 a, and2142 b are not necessarily provided. As illustrated in FIG. 9C, astructure where the oxide semiconductor layers 2141 a, 2141 b, 2142 a,and 2142 b are not provided may be employed. However, by the provisionof the oxide semiconductor layers 2141 a, 2141 b, 2142 a, and 2142 b, ajunction between an upper electrode and the first oxide semiconductorlayer can be satisfactory, and thermally stable operation can beperformed as compared to a Schottky junction. Moreover, favorablemobility can be maintained at a high drain voltage.

The electrodes 215 to 217 function as a source electrode or a drainelectrode. The electrodes 215 to 217 preferably have a single-layerstructure or a layered structure using an element such as aluminum,copper, chromium, silicon, titanium, neodymium, scandium, or molybdenumor an aluminum alloy to which an element for preventing hillocks isadded. Further, when heat treatment of 200° C. to 600° C. is performed,the conductive film preferably has heat resistance enough to withstandthe heat treatment. For example, when a layered structure of a titaniumfilm, an aluminum film, and a titanium film is employed for theelectrodes 215 to 217, the electrodes 215 to 217 have low resistance andhillocks are not likely to occur in the aluminum film. The electrodes215 to 217 can be formed by a sputtering method or a vacuum evaporationmethod. Alternatively, the electrodes 215 to 217 may be formed bydischarging a conductive nanopaste of silver, gold, copper, or the likeby a screen printing method, an ink jet method, or the like and bakingthe nanopaste.

The reduction prevention layer 218 is provided at least over a region(also referred to as a back channel region) in the oxide semiconductorlayer 2132 between the electrodes 216 and 217, and has functions ofpreventing impurities such as moisture from entering the oxidesemiconductor layer 2132 and preventing reduction of the back channelregion. As the reduction prevention layer 218, a non-reducible film suchas an oxide film made of silicon oxide, aluminum oxide, or the like canbe used, for example. Note that the reduction prevention layer 218should have a function of preventing reduction as one of its functions,and another function can be added to the reduction prevention layer 218.

Note that as the transistor 202 in the logic circuit shown in thisembodiment, a thin film transistor can be used in which the thresholdvoltage is shifted by predetermined processing on a back channel regionso that the thin film transistor is an enhancement transistor.Processing for controlling the density of oxygen vacancies (alsoreferred to as oxide vacancy defects) is an example of the predeterminedprocessing (such processing is also referred to as oxygen vacancycontrol processing). Examples of the oxygen vacancy control processingare oxygen plasma treatment, annealing treatment under an oxygen stream,and oxygen ion irradiation treatment. For example, the oxygen plasmatreatment refers to treatment in which a surface of an oxidesemiconductor layer is treated with radicals generated by glow dischargeplasma of an oxygen gas, and instead of using only oxygen, a mixture gasof an oxygen gas and a rare gas may be employed as the gas used forgenerating plasma. By using the thin film transistor, a logic circuitusing a plurality of transistors whose threshold voltages are differentfrom each other can be formed more easily even when a transistor usingan oxide semiconductor is used. The density of oxygen vacancies of thetransistors 201 and 202 are made different from each other with theoxygen vacancy control processing, so that a logic circuit includingboth a depletion transistor and an enhancement transistor can be formed.

In addition, in the logic circuit in this embodiment, one of a sourceelectrode and a drain electrode of one transistor may be directlyconnected to a gate electrode of another transistor. The logic circuitwith such a structure will be described with reference to FIGS. 10A and10B. FIGS. 10A and 10B illustrate a structure of the logic circuit inthis embodiment. FIG. 10A is a top view of the logic circuit. FIG. 10Bis a cross-sectional view of the logic circuit along Z1-Z2 in FIG. 10A.Note that in the logic circuit illustrated in FIGS. 10A and 10B, thedescription of the logic circuit illustrated in FIGS. 9A to 9C isemployed as appropriate for the same portion as the logic circuit inFIGS. 9A to 9C.

Like the logic circuit in FIGS. 9A to 9C, the logic circuit in FIGS. 10Aand 10B includes the transistors 201 and 202. Further, in the transistor201 of the logic circuit in FIGS. 10A and 10B, the gate electrode 2111is directly connected to the electrode 216 through an opening portionprovided in the gate insulating layer 212.

In the logic circuit using the transistor in which the gate electrode2112 and the electrode 216 are connected through the opening portionprovided in the gate insulating layer 212 as described above,satisfactory contact can be obtained, and contact resistance can bereduced. Accordingly, the number of openings can be reduced, whichresults in reducing the area occupied by the logic circuit.

As described above, a logic circuit including transistors whosethreshold voltages are different from each other can be provided byusing thin film transistors including an oxide semiconductor. Moreover,by using the thin film transistors including the oxide semiconductor,the logic circuit can operate at high speed. Further, since the logiccircuit can be formed using transistors of the same conductivity type,the process can be simplified as compared to that of a logic circuitusing transistors of different conductivity types.

Embodiment 2

In this embodiment, a shift register using the logic circuit illustratedin FIG. 3 in Embodiment 1 as a unit sequential logic circuit will bedescribed. Note that in this embodiment, the case where the logiccircuit in FIG. 3 serves as the unit sequential logic circuit isdescribed as an example.

The shift register in this embodiment includes a plurality of logiccircuits in FIG. 3 in Embodiment 1 as unit sequential logic circuits,and a plurality of unit sequential logic circuits are electricallyconnected to each other in series. A specific structure will bedescribed with reference to FIG. 11. FIG. 11 is a circuit diagramillustrating a structure of the shift register in this embodiment.

The shift register illustrated in FIG. 11 includes a logic circuit 3011,a logic circuit 3012, a logic circuit 3013, a NAND circuit 3140, a NANDcircuit 3141, a NAND circuit 3142, and a NAND circuit 3143. Note thatalthough FIG. 11 illustrates three (also referred to as three-stage)unit sequential logic circuits, one embodiment of the invention is notlimited thereto and may include at least two-stage unit sequential logiccircuits.

The logic circuit 3011 includes a transistor 3111, an inverter 3121A, aninverter 3122A, an inverter 3123A, and a transistor 3131. The logiccircuit 3011 has the same circuit configuration as the logic circuit inFIG. 3. Specifically, the transistor 3111 corresponds to the transistor111; the inverter 3121A, the inverter 1121; the inverter 3122A, theinverter 1122; the inverter 3123A, the inverter 1123; and the transistor3131, the transistor 113. Therefore, the description of the logiccircuit in FIG. 3 is employed as appropriate for each element. Moreover,in the logic circuit 3011, a first clock signal is input to a gate ofthe transistor 3111, and a second clock signal is input to a gate of thetransistor 3131.

The logic circuit 3012 includes a transistor 3112, an inverter 3121B, aninverter 3122B, an inverter 3123B, and a transistor 3132. The logiccircuit 3012 has the same configuration as the logic circuit in FIG. 3.Specifically, the transistor 3112 corresponds to the transistor 111; theinverter 3121B, the inverter 1121; the inverter 3122B, the inverter1122; the inverter 3123B, the inverter 1123; and the transistor 3132,the transistor 113. Therefore, the description of the logic circuit inFIG. 3 is employed as appropriate for each element. Moreover, in thelogic circuit 3012, the second clock signal is input to a gate of thetransistor 3112, and the first clock signal is input to a gate of thetransistor 3132.

The logic circuit 3013 includes a transistor 3113, an inverter 3121C, aninverter 3122C, an inverter 3123C, and a transistor 3133. The logiccircuit 3013 has the same configuration as the logic circuit in FIG. 3.Specifically, the transistor 3113 corresponds to the transistor 111; theinverter 3121C, the inverter 1121; the inverter 3122C, the inverter1122; the inverter 3123C, the inverter 1123; and the transistor 3133,the transistor 113. Therefore, the description of the logic circuit inFIG. 3 is employed as appropriate for each element. Moreover, in thelogic circuit 3013, the first clock signal is input to a gate of thetransistor 3113, and the second clock signal is input to a gate of thetransistor 3133.

An output terminal of the inverter 3123A in the logic circuit 3011 iselectrically connected to one of a source and a drain of the transistor3112 in the logic circuit 3012. An output terminal of the inverter 3123Bin the logic circuit 3012 is electrically connected to one of a sourceand a drain of the transistor 3113 in the logic circuit 3013.

Further, in the logic circuit 3011, one of a source and a drain of thetransistor 3111 is electrically connected to a first input terminal ofthe NAND circuit 3140, and the output terminal of the inverter 3123A iselectrically connected to a second input terminal of the NAND circuit3140 and a first input terminal of the NAND circuit 3141. In the logiccircuit 3012, one of the source and the drain of the transistor 3112 iselectrically connected to the second input terminal of the NAND circuit3140 and the first input terminal of the NAND circuit 3141, and theoutput terminal of the inverter 3123B is electrically connected to asecond input terminal of the NAND circuit 3141 and a first inputterminal of the NAND circuit 3142. In the logic circuit 3013, one of thesource and the drain of the transistor 3113 is electrically connected tothe second input terminal of the NAND circuit 3141 and the first inputterminal of the NAND circuit 3142, and an output terminal of theinverter 3123C is electrically connected to a second input terminal ofthe NAND circuit 3142 and a first input terminal of the NAND circuit3143.

Each of the NAND circuits 3140 to 3143 can be constituted by transistorshaving the same conductivity type as the transistors included in thelogic circuits. By using transistors of the same conductivity type, theNAND circuit can be formed in the same process as the logic circuit, andthus can be easily formed. A circuit configuration of a NAND circuitincluding transistors of the same conductivity type will be describedwith reference to FIG. 12. FIG. 12 is a circuit diagram illustrating acircuit configuration of a NAND circuit in this embodiment.

The NAND circuit illustrated in FIG. 12 includes a transistor 321, atransistor 322, and a transistor 323.

The transistor 321 is a depletion transistor. One of a source and adrain of the transistor 321 is electrically connected to a power supplyline 325, and a high power supply voltage is applied to one of thesource and the drain. A gate and the other of the source and the drainof the transistor 321 are electrically connected to each other

The transistor 322 is an enhancement transistor. One of a source and adrain of the transistor 322 is electrically connected to the other ofthe source and the drain of the transistor 321.

The transistor 323 is an enhancement transistor. One of a source and adrain of the transistor 323 is electrically connected to the other ofthe source and the drain of the transistor 322. The other of the sourceand the drain of the transistor 323 is electrically connected to a powersupply line 324, and a low power supply voltage is applied to the otherof the source and the drain of the transistor 323.

In the logic circuit in this embodiment, a first input signal is inputto a gate of the transistor 323, a second input signal is input to agate of the transistor 322, and a voltage (V326) of a node 326 betweenthe transistor 322 and the transistor 321 is output as an output signal.

Next, operation of the NAND circuit illustrated in FIG. 12 will bedescribed.

The operation of the NAND circuit in FIG. 12 can be classified into twocategories depending on whether at least one of a voltage (Vin1) of thefirst input signal and a voltage (Vin2) of the second input signal islow or the voltages of the first and second input signals are high. Bothcases will be described with reference to FIGS. 13A and 13B. FIGS. 13Aand 13B illustrate operation of the NAND circuit in this embodiment.Note that in this embodiment, the case where data is 0 in a low stateand data is 1 in a high state is described as an example; however, oneembodiment of the invention is not limited thereto, and data can be 1 ina low state and can be 0 in a high state.

FIG. 13A illustrates the operation in the case where Vin1=VH andVin2=VL, the case where Vin1=VL and Vin2=VH, and the case where Vin1=VLand Vin2=VL. At this time, one or both of the transistors 322 and 323is/are turned off, and the resistance (R322+R323) of the transistors 322and 323 is higher than the resistance (R321) of the transistor 321, thatis, (R322+R323)>R321; accordingly, V326 is VH, and a voltage (Vout) ofthe output signal is VH.

FIG. 13B illustrates the operation in the case where Vin1=VH andVin2=VH. At this time, the transistors 321 and 322 are turned on, and itfollows that R322+R323<R321; accordingly, V326 is VL, and Vout is VL.The above is the operation of the NAND circuit illustrated in FIG. 12.

When the NAND circuit is formed using transistors of the sameconductivity type as described above, it can be formed in the sameprocess as another logic circuit. Moreover, one embodiment of theinvention is not limited to the structure in FIG. 12, and the NANDcircuit can have another structure if it can have the same function.

Next, operation of the shift register illustrated in FIG. 11 will bedescribed with reference to FIG. 14. FIG. 14 is a timing chartillustrating the operation of the shift register in FIG. 11.

In the shift register in FIG. 11, the operations of the logic circuit,which are illustrated in FIGS. 4A and 4B, FIGS. 5C and 5D, and FIG. 6,are sequentially performed in each of the logic circuits 3011 to 3013.For the operation of each logic circuit, the description of theoperation of the logic circuit illustrated in FIGS. 4A and 4B, FIGS. 5Cand 5D, and FIG. 6 is employed as appropriate.

The operation of the shift register in this embodiment is classifiedinto ten periods as illustrated in FIG. 14. In a first period, a voltageVin of an input signal to the logic circuit 3011 is VH. In a secondperiod and a third period, a voltage (V3171) of a node 3171 between thelogic circuit 3011 and the logic circuit 3012 is changed from VH to VL.Further, in the third period and a fourth period, a voltage of an outputsignal from the NAND circuit 3140 is VH.

In the fourth period and a fifth period, a voltage of an input signal tothe logic circuit 3012 (an output signal from the logic circuit 3011) ischanged from VL to VH. In the fifth period and a sixth period, a voltage(V3172) of a node 3172 between the logic circuit 3012 and the logiccircuit 3013 is changed from VH to VL. In the sixth period and a seventhperiod, a voltage of an output signal from the NAND circuit 3141 is VH.

In the seventh period and an eighth period, a voltage of an input signalto the logic circuit 3013 (an output signal from the logic circuit 3012)is changed from VL to VH. In the eighth period and a ninth period, avoltage (V3173) of a node 3173 between the logic circuit 3013 and anext-stage logic circuit is changed from VH to VL. In the ninth periodand a tenth period, a voltage of an output signal from the NAND circuit3142 is VH.

When another logic circuit is connected to an output terminal of thelogic circuit 3013, a voltage of an input signal is changed from VL toVH in a given period and a voltage of an output signal is changed to VHin another given period as described above. Moreover, in a period wherea voltage of the output signal from another logic circuit is VL, avoltage of an output signal from the NAND circuit 3143 is VH.

As described above, a shift register can be constituted by logiccircuits including TFTs using an oxide semiconductor. The TFT using theoxide semiconductor has higher mobility than a conventional TFT usingamorphous silicon; therefore, by applying the TFT using the oxidesemiconductor to the shift register, the shift register can operate athigh speed.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 3

In this embodiment, a logic circuit including a transistor having astructure different from that in the above embodiment will be described.

A logic circuit which is one embodiment of the invention disclosed inthis specification can be formed using not only transistors with thestructures illustrated in FIGS. 9A to 9C but also transistors withanother structure. A logic circuit to which a transistor with anotherstructure is applied will be described with reference to FIGS. 15A and15B. FIGS. 15A and 15B illustrate a structure of the logic circuit inthis embodiment. FIG. 15A is a top view, and FIG. 15B is across-sectional view of the logic circuit along Z1-Z2 in FIG. 15A. Notethat in the logic circuit illustrated in FIGS. 15A and 15B, thedescription of the logic circuit illustrated in FIGS. 9A to 9C isemployed as appropriate for the same portion as the logic circuit inFIGS. 9A to 9C.

Like the logic circuit in FIGS. 9A to 9C, the logic circuit in FIGS. 15Aand 15B includes the transistor 201 and the transistor 202.

Moreover, in the transistor 201 of the logic circuit in FIGS. 15A and15B, the gate electrode 2111 is provided over the substrate 210. Thegate insulating layer 212 is provided over the gate electrode 2111. Theelectrodes 215 and 216, which are a pair of electrodes, are providedover the gate insulating layer 212. The oxide semiconductor layers 2141a and 2141 b are provided over the electrodes 215 and 216. The oxidesemiconductor layer 2131 is provided over the gate insulating layer 212and the electrodes 215 and 216.

In the transistor 202, the gate electrode 2112 is provided over thesubstrate 210. The gate insulating layer 212 is provided over the gateelectrode 2112. The electrodes 216 and 217, which are a pair ofelectrodes, are provided over the gate insulating layer 212. The oxidesemiconductor layers 2142 a and 2142 b are provided over the electrodes216 and 217. The oxide semiconductor layer 2132 is provided over thegate insulating layer 212, the oxide semiconductor layers 2142 a and2142 b, and the electrodes 216 and 217. The reduction prevention layer218 is provided over a region in the oxide semiconductor layer 2132between the electrodes 216 and 217. Note that the oxide semiconductorlayers 2141 a and 2141 b correspond to the oxide semiconductor layers2141 a and 2141 b in the logic circuit illustrated in FIGS. 9A and 9B,and the oxide semiconductor layers 2142 a and 2142 b correspond to theoxide semiconductor layers 2142 a and 2142 b in the logic circuitillustrated in FIGS. 9A and 9B.

The logic circuit illustrated in FIGS. 15A and 15B includes a transistorin which the oxide semiconductor layers 2131 and 2132 are formed overthe electrodes 215 to 217 and the oxide semiconductor layers 2141 a,2141 b, 2142 a, and 2142 b (such a structure is also referred to as abottom-contact type). When the logic circuit which is one embodiment ofthe invention disclosed in this specification is formed using abottom-contact transistor, the area where the oxide semiconductor layerand the electrode are in contact with each other can be increased, sothat peeling or the like can be prevented.

In addition, as the transistor 202 in the logic circuit in FIGS. 15A and15B, a thin film transistor can be used in which the threshold voltageis shifted by predetermined processing on a back channel region so thatthe thin film transistor is an enhancement transistor, as in the logiccircuit in FIGS. 9A to 9C. The processing shown in Embodiment 1 can beapplied to the predetermined processing.

Note that the oxide semiconductor layers 2141 a, 2141 b, 2142 a, and2142 b are provided in the logic circuit in FIGS. 15A and 15B as in thelogic circuit in FIGS. 9A and 9B; however, one embodiment of theinvention is not limited thereto, and a structure where the oxidesemiconductor layers 2141 a, 2141 b, 2142 a, and 2142 b are not providedmay be employed.

Further, in the logic circuit in FIGS. 15A and 15B, the gate electrode2112 of the transistor 202 and the electrode 216 can be in contact witheach other through an opening portion provided in the gate insulatinglayer 212, as in the logic circuit illustrated in FIGS. 10A and 10B.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 4

In this embodiment, a logic circuit including a transistor having astructure different from those in the above embodiments will bedescribed.

A logic circuit can be formed using not only transistors with thestructures illustrated in FIGS. 9A to 9C and FIGS. 15A and 15B but alsotransistors with another structure. A logic circuit to which atransistor with a structure which is different from those in FIGS. 9A to9C and FIGS. 15A and 15B is applied will be described with reference toFIGS. 16A and 16B. FIGS. 16A and 16B illustrate a structure of the logiccircuit in this embodiment. FIG. 16A is a top view, and FIG. 16B is across-sectional view along Z1-Z2 in FIG. 16A. Note that in the logiccircuit illustrated in FIGS. 16A and 16B, the description of the logiccircuit illustrated in FIGS. 9A to 9C is employed as appropriate for thesame portion as the logic circuit in FIGS. 9A to 9C.

Like the logic circuit in FIGS. 9A to 9C, the logic circuit in FIGS. 16Aand 16B includes the transistor 201 and the transistor 202.

In the transistor 201 of the logic circuit in FIGS. 16A and 16B, thegate electrode 2111 is provided over the substrate 210. The gateinsulating layer 212 is provided over the gate electrode 2111. The oxidesemiconductor layer 2131 is provided over the gate insulating layer 212.A buffer layer 2191 is provided over part of the oxide semiconductorlayer 2131. The oxide semiconductor layers 2141 a and 2141 b areprovided over the oxide semiconductor layer 2131 and the buffer layer2191. The electrodes 215 and 216, which are a pair of electrodes, areprovided over the oxide semiconductor layers 2141 a and 2141 b,respectively.

In the transistor 202, the gate electrode 2112 is provided over thesubstrate 210. The gate insulating layer 212 is provided over the gateelectrode 2112. The oxide semiconductor layer 2132 is provided over thegate insulating layer 212. A buffer layer 2192 is provided over a regionin the oxide semiconductor layer 2132 between the electrodes 216 and217. The oxide semiconductor layers 2142 a and 2142 b are provided overthe oxide semiconductor layer 2132 and the buffer layer 2192. Theelectrodes 216 and 217, which are a pair of electrodes, are providedover the oxide semiconductor layers 2142 a and 2142 b, respectively.

For the buffer layers 2191 and 2192, an inorganic material (e.g.,silicon oxide, silicon nitride, silicon oxynitride, or silicon nitrideoxide) can be used. Alternatively, a photosensitive ornon-photosensitive organic material (organic resin material, forexample, polyimide, acrylic, polyamide, polyimideamide, resist, orbenzocyclobutene), a film made of plural kinds of these materials, or alayered film of such films can be used, or siloxane may be used. As amethod for manufacturing the buffer layers 2191 and 2192, a vapordeposition method such as a plasma CVD method or a thermal CVD method,or a sputtering method can be used. Alternatively, a coating method suchas a spin coating method, a droplet discharging method, or a printingmethod (such as screen printing or offset printing by which a pattern isformed), which is a wet process, may be used. The buffer layers 2191 and2192 may be formed in such a manner that a film is deposited and thenetched so that the shape is processed, or may be selectively formed by adroplet discharging method or the like.

The logic circuit illustrated in FIGS. 16A and 16B includes a transistorin which the buffer layer is provided (such a structure is also referredto as a channel-stop type). For example, when the buffer layer is formedusing a non-reducible film (formed of silicon oxide or aluminum oxide,for example), the buffer layer can function as a reduction preventionlater; accordingly, the logic circuit which is one embodiment of theinvention disclosed in this specification can be formed using atransistor with the same structure as a conventional channel-stoptransistor.

In addition, as the transistor 202 in the logic circuit in FIGS. 16A and16B, a thin film transistor can be used in which the threshold voltageis shifted by predetermined processing on a back channel region so thatthe thin film transistor is an enhancement transistor. The processingshown in Embodiment 1 can be applied to the predetermined processing.

Note that the oxide semiconductor layers 2141 a, 2141 b, 2142 a, and2142 b are provided in the logic circuit in FIGS. 16A and 16B as in thelogic circuit in FIGS. 9A and 9B; however, one embodiment of theinvention is not limited thereto, and a structure where the oxidesemiconductor layers 2141 a, 2141 b, 2142 a, and 2142 b are not providedmay be employed.

Further, in the logic circuit in FIGS. 16A and 16B, the gate electrode2112 of the transistor 202 and the electrode 216 can be in contact witheach other through an opening portion provided in the gate insulatinglayer 212, as in the logic circuit illustrated in FIGS. 10A and 10B.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 5

In this embodiment, a method for manufacturing a logic circuit will bedescribed. Note that in this embodiment, a method for manufacturing thelogic circuit illustrated in FIGS. 9A and 9B is described as an example.

A method for manufacturing a logic circuit in this embodiment will bedescribed with reference to FIGS. 17A and 17B and FIGS. 18C and 18D.FIGS. 17A and 17B and FIGS. 18C and 18D are cross-sectional viewsillustrating a method for manufacturing the logic circuit in thisembodiment.

First, as illustrated in FIG. 17A, a first conductive film is formedover the substrate 210. The first conductive film is selectively etchedusing a first photomask so that the gate electrodes 2111 and 2112 areformed. Then, the gate insulating layer 212 is formed over the gateelectrodes 2111 and 2112. The first conductive film can be formed by asputtering method, for example. The gate insulating layer 212 can beformed by a plasma CVD method or a sputtering method. At this time, thegate electrodes 2111 and 2112 are preferably formed to be tapered.

Next, a first oxide semiconductor film is formed over the gateinsulating layer 212, and a second oxide semiconductor film is formedthereover. The first oxide semiconductor film can be formed by asputtering method, for example. Note that before the first oxidesemiconductor film is formed, reverse sputtering in which plasma isgenerated by introduction of an argon gas is preferably performed toremove dust attached to a surface of the gate insulating layer 212 and abottom surface of an opening portion. The reverse sputtering is a methodin which voltage is applied to the substrate side, not to the targetside, using an RF power supply in an argon atmosphere to generate plasmaon the substrate so that a surface of the substrate is modified. Notethat nitrogen, helium, or the like may be used instead of the argonatmosphere. Further, the reverse sputtering may be performed in anatmosphere where oxygen, hydrogen, N₂O, or the like is added to theargon atmosphere or in an atmosphere where Cl₂, CF₄, or the like isadded to the argon atmosphere.

Next, the first and second oxide semiconductor films are etched using asecond photomask, and then, a second conductive film is formed. Thesecond conductive film can be formed by a sputtering method, forexample. Moreover, the second conductive film is selectively etchedusing a third photomask, so that the electrodes 215, 216, and 217 areformed as illustrated in FIG. 17B. Note that before the secondconductive film is formed, reverse sputtering in which plasma isgenerated by introduction of an argon gas is preferably performed toremove dust attached to the surface of the gate insulating layer 212 andthe etched oxide semiconductor layers.

Note that when the second conductive film is etched, the first andsecond oxide semiconductor layers are partly etched. Accordingly, asillustrated in FIG. 15B, the oxide semiconductor layers 2131 and 2132are formed over the gate insulating layer 212, the oxide semiconductorlayers 2141 a and 2141 b are formed over the oxide semiconductor layer2131, and the oxide semiconductor layers 2142 a and 2142 b are formedover the oxide semiconductor layer 2132. By this etching, portions ofthe oxide semiconductor layers 2131 and 2132, which overlap with thegate electrodes 2111 and 2112, are made thinner.

Wet etching or dry etching is used as an etching method at this time.For example, when an aluminum film or an aluminum alloy film is used asthe second conductive film, wet etching can be performed using asolution in which phosphoric acid, acetic acid, and nitric acid aremixed. In this etching step, the oxide semiconductor layers 2131 and2132 are also partly etched. Further, since the oxide semiconductorlayers 2141 a, 2141 b, 2142 a, and 2142 b and the electrodes 215 to 217are etched at one time, the edges of the oxide semiconductor layers 2141a, 2141 b, 2142 a, and 2142 b and the electrodes 215 to 217 are aligned,so that a smooth side surface is formed. Moreover, in the case of usingwet etching, etching is isotropically performed, and the edges of theelectrodes 215 to 217 are recessed with respect to the edge of a resistmask.

In addition, in the method for manufacturing the logic circuit in thisembodiment, as an example, oxygen vacancy control processing isperformed on an oxide semiconductor layer in a transistor functioning asan enhancement transistor (the oxide semiconductor layer 2132 in thisembodiment). As illustrated in FIG. 18C, the oxygen vacancy controlprocessing is performed, so that an oxygen vacancy control region 250with a low oxygen vacancy density is formed between the electrodes 216and 217 over a surface of the oxide semiconductor layer 2132, which isopposite to a surface in contact with the gate insulating layer 212. Inthis embodiment, oxygen plasma treatment is performed as an example ofthe oxygen vacancy control processing. The processing conditions are setas appropriate so that the threshold voltage of a transistor to beformed is positive.

Note that in FIG. 18C, the oxygen plasma treatment should be performedat least on the oxide semiconductor layer 2132 and not necessarily onthe oxide semiconductor layer 2131. For example, when only the oxidesemiconductor layer 2132 is to be subjected to the oxygen plasmatreatment, the oxygen plasma treatment may be performed after a mask isformed over the oxide semiconductor layer 2131. Further, when the oxygenplasma treatment is performed on the oxide semiconductor layer 2131, thethreshold voltage is shifted to a positive value; however, the thresholdvoltage is shifted due to the shift of the threshold voltage over timeif a reduction prevention layer is not provided over the oxidesemiconductor layer 2131. Thus, both a depletion transistor and anenhancement transistor can be manufactured. Moreover, when the oxygenplasma treatment is performed also on the oxide semiconductor layer2131, an additional mask is not necessary, so that the process can besimplified.

Next, heat treatment is performed in the air or a nitrogen atmosphere.The heat treatment is preferably performed at 200° C. to 600° C.,typically 300° C. to 500° C. With the heat treatment, atoms in the oxidesemiconductor film are rearranged. Since distortion which preventscarrier transfer is eliminated by the heat treatment, the heat treatment(including light annealing) performed here is important. Note that thereis no particular limitation on the timing when the heat treatment isperformed as long as the heat treatment is performed after the oxidesemiconductor film is formed, and the heat treatment can be performedany time after the semiconductor film is formed.

Then, as illustrated in FIG. 18D, the reduction prevention layer 218 isformed over a region between the electrodes 216 and 217, which includesthe oxygen vacancy control region 250 in the oxide semiconductor layer(the oxide semiconductor layer 2132 in FIG. 18D) of the transistor whichis to function as the enhancement transistor later. The reductionprevention layer 218 is formed only over the oxide semiconductor layerof the transistor functioning as the enhancement transistor, whereby atransistor including the semiconductor layer where the reductionprevention layer 218 is not provided serves as a depletion transistor;accordingly, transistors having different threshold voltages can beformed over one substrate. The reduction prevention layer 218 can beformed by a sputtering method, for example.

Note that the above-described order of steps is an example, and there isno particular limitation on the order of steps. For example, althoughone additional photomask needs to be used, etching may be performed insuch a manner that the second conductive film is etched using onephotomask and part of the oxide semiconductor layer and part of theoxide semiconductor film are etched using another photomask.

Alternatively, instead of performing the oxygen plasma treatment, thereduction prevention layer 218 may be formed by a sputtering method inFIG. 18D without the oxygen plasma treatment in FIG. 18C. This isbecause oxygen is used as a gas when the reduction prevention layer 218is formed by a sputtering method, so that advantageous effect similar tothose of the oxygen plasma treatment can be obtained.

By the above method, the logic circuit illustrated in FIGS. 9A and 9Bcan be formed. Moreover, by using the manufacturing method in thisembodiment, a logic circuit using transistors which have differentthreshold voltages and are formed over one substrate can be formed.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 6

In this embodiment, a display device will be described as an example ofa device to which the logic circuit shown in the above embodiments canbe applied.

The logic circuits shown in the above embodiments can be applied to avariety of display devices such as a liquid crystal display device andan electroluminescent display device. A structure of a display device inthis embodiment will be described with reference to FIG. 19. FIG. 19 isa block diagram illustrating a structure of the display device in thisembodiment.

As illustrated in FIG. 19, the display device in this embodimentincludes a pixel portion 701, a scan line driver circuit 702, and asignal line driver circuit 703.

The pixel portion 701 includes a plurality of pixels 704 and has a dotmatrix structure. Specifically, the plurality of pixels 704 are arrangedin the row and column directions. Each pixel 704 is electricallyconnected to the scan line driver circuit 702 through a scan line andelectrically connected to the signal line driver circuit 703 through asignal line. Note that in FIG. 19, the scan line and the signal line arenot illustrated for simplification.

The scan line driver circuit 702 is a circuit for selecting the pixel704 to which a data signal is input, and outputs a selection signal tothe pixel 704 through the scan line.

The signal line driver circuit 703 is a circuit for outputting datawritten to the pixel 704 as a signal, and outputs pixel data as a signalthrough the signal line to the pixel 704 selected by the scan linedriver circuit 702.

The pixel 704 includes at least a display element and a switchingelement. A liquid crystal element or a light-emitting element such as anEL element can be applied to the display element, for example. Atransistor can be applied to the switching element, for example.

Next, an example of structures of the scan line driver circuit 702 andthe signal line driver circuit 703 will be described with reference toFIGS. 20A and 20B. FIGS. 20A and 20B are block diagrams eachillustrating a structure of the driver circuit. FIG. 20A is a blockdiagram illustrating a structure of the scan line driver circuit. FIG.20B is a block diagram illustrating a structure of the signal linedriver circuit.

As illustrated in FIG. 20A, the scan line driver circuit 702 includes ashift register 900, a level shifter 901, and a buffer 902.

Signals such as a gate start pulse (GSP) and a gate clock signal (GCK)are input to the shift register 900, and selection signals aresequentially output from sequential logic circuits. Moreover, the shiftregister shown in Embodiment 2 can be applied to the shift register 900.

Further, as illustrated in FIG. 20B, the signal line driver circuit 703includes a shift register 903, a first latch circuit 904, a second latchcircuit 905, a level shifter 906, and a buffer 907.

A signal such as a start pulse (SSP) is input to the shift register 903,and selection signals are sequentially output from the sequential logiccircuits.

A data signal is input to the first latch circuit 904. The first latchcircuit can be constituted by one or more of the logic circuits shown inthe above embodiments, for example.

The buffer 907 has a function of amplifying a signal and includes anoperational amplifier or the like. The buffer 907 can be constituted byone or more of the logic circuits shown in the above embodiments, forexample.

The second latch circuit 905 can hold a latch (LAT) signal temporallyand outputs the held latch signals all at once to the pixel portion 701in FIG. 19. This is referred to as line sequential driving. Therefore,in the case of using a pixel in which not line sequential driving butdot sequential driving is performed, the second latch circuit 905 is notnecessary. The second latch circuit 905 can be constituted by one ormore of the logic circuits shown in the above embodiments, for example.

Next, operation of the display device illustrated in FIG. 19 will bedescribed.

First, a scan line is selected by the scan line driver circuit 702. Tothe pixel 704 connected to the selected scan line, a data signal isoutput from the signal line driver circuit 703 through a signal line bya signal input from the scan line driver circuit 702. Accordingly, datais written to the pixel 704, and the pixel 704 enters into a displaystate. Scan lines are selected by the scan line driver circuit 702, anddata is written to all the pixels 704. The above is the operation of thedisplay device in this embodiment.

The circuits in the display device illustrated in FIG. 19 can all beprovided over one substrate, or can be constituted by transistors of thesame conductivity type. By providing the circuits over one substrate,the size of the display device can be reduced. By using transistors ofthe same conductivity type, the process can be simplified.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 7

In this embodiment, a liquid crystal display device will be described asan example of the display device shown in Embodiment 6.

An example of a circuit configuration of a pixel in a display device inthis embodiment will be described with reference to FIG. 21. FIG. 21 isa circuit diagram illustrating a circuit configuration of a pixel in thedisplay device in this embodiment.

As illustrated in FIG. 21, the pixel includes a transistor 821, a liquidcrystal element 822, and a storage capacitor 823.

The transistor 821 functions as a selection switch. A gate of thetransistor 821 is electrically connected to a scan line 804, and one ofa source and a drain thereof is electrically connected to a signal line805.

The liquid crystal element 822 has a first terminal and a secondterminal. The first terminal is electrically connected to the other ofthe source and the drain of the transistor 821. A ground potential or avoltage with a given value is applied to the second terminal. The liquidcrystal element 822 includes a first electrode which serves as part orall of the first terminal, a second electrode which serves as part orall of the second terminal, and a layer including liquid crystalmolecules whose transmittance is changed by applying voltage between thefirst electrode and the second electrode (such a layer is referred to asa liquid crystal layer).

The storage capacitor 823 has a first terminal and a second terminal.The first terminal is electrically connected to the other of the sourceand the drain of the transistor 821. The ground potential or a voltagewith a given value is applied to the second terminal. The storagecapacitor 823 includes a first electrode which serves as part or all ofthe first terminal, a second electrode which serves as part or all ofthe second terminal, and a dielectric layer. Note that although thestorage capacitor 823 is not necessarily provided, the provision of thestorage capacitor 823 can reduce adverse effects due to leakage currentof the transistor 821.

Note that for the display device in this embodiment, a TN (twistednematic) mode, an IPS (in-plane-switching) mode, an FFS (fringe fieldswitching) mode, an MVA (multi-domain vertical alignment) mode, a PVA(patterned vertical alignment) mode, an ASM (axially symmetric alignedmicro-cell) mode, an OCB (optical compensated birefringence) mode, anFLC (ferroelectric liquid crystal) mode, an AFLC (antiferroelectricliquid crystal) mode, or the like can be used.

Alternatively, blue-phase liquid crystal for which an alignment film isnot necessary may be used. The blue phase is a kind of liquid crystalphase and appears just before phase transition from a cholesteric phaseto an isotropic phase when temperature of cholesteric liquid crystalrises. Since the blue phase appears only in a narrow temperature range,a liquid crystal composition in which 5 wt. % or more of a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. As for the liquid crystal compositionwhich contains blue-phase liquid crystal and the chiral material, theresponse speed is as high as 10 μs to 100 μs, alignment treatment is notnecessary due to optical isotropy, and viewing angle dependence is low.

Next, operation of the pixel illustrated in FIG. 21 will be described.

First, a pixel to which data is written is selected, and the transistor821 in the selected pixel is turned on by a signal input from the scanline 804.

At this time, a data signal from the signal line 805 is input throughthe transistor 821, so that the first terminal of the liquid crystalelement 822 has the same voltage as the data signal, and thetransmittance of the liquid crystal element 822 is set depending onvoltage applied between the first terminal and the second terminal.After data writing, the transistor 821 is turned off by a signal inputfrom the scan line 804, the transmittance of the liquid crystal element822 is maintained during a display period, and the pixel enters into adisplay state. The above operation is sequentially performed per scanline 804, and the above operation is performed in all the pixels. Theabove is the operation of the pixel.

In displaying moving images in a liquid crystal display device, there isa problem in that an afterimage or motion blur occurs because of slowresponse of liquid crystal molecules themselves. In order to improvemoving image characteristics of the liquid crystal display device, thereis a driving technique called black insertion, in which the entirescreen is displayed as black every other frame.

Moreover, there is a driving technique called double-frame rate driving,in which a vertical period is 1.5 times or 2 times or more as long as anormal vertical period in order to increase the response speed, and graylevel to be written is selected for a plurality of divided fields ineach frame.

Further, in order to improve moving image characteristics of the liquidcrystal display device, there is a driving technique in which aplurality of LED (light-emitting diode) light sources, a plurality of ELlight sources, or the like are used as backlights to form an area lightsource, and the light sources forming the area light source areindependently lit intermittently in one frame period. For the area lightsource, LEDs of three kinds or more or an LED which emits white lightmay be used. Since a plurality of LEDs can be independently controlled,the timing when the LED emits light can be synchronized with the timingwhen optical modulation of the liquid crystal layer is changed. Part ofthe LEDs can be turned off in this driving technique, so that powerconsumption can be reduced particularly in the case of displaying animage in which a black display region occupies a large area in onescreen.

By combining these driving techniques, display characteristics such asmoving image characteristics of the liquid crystal display device can beimproved as compared to those of a conventional liquid crystal displaydevice.

Next, a structure of a display device in this embodiment, which includesthe above pixel, will be described with reference to FIGS. 22A and 22B.FIGS. 22A and 22B illustrate a structure of the pixel in the displaydevice in this embodiment. FIG. 22A is a top view, and FIG. 22B is across-sectional view. Note that dotted lines A1-A2 and B1-B2 in FIG. 22Acorrespond to cross sections A1-A2 and B1-B2 in FIG. 22B, respectively.

As illustrated in FIGS. 22A and 22B, the display device in thisembodiment includes, in the cross section A1-A2, a gate electrode 2001over a substrate 2000; a gate insulating layer 2002 provided over thegate electrode 2001; an oxide semiconductor layer 2003 provided over thegate insulating layer 2002; a pair of oxide semiconductor layers 2004 aand 2004 b provided over the oxide semiconductor layer 2003; electrodes2005 a and 2005 b provided so as to be in contact with the oxidesemiconductor layers 2004 a and 2004 b; a protective insulating layer2007 provided over the electrodes 2005 a and 2005 b and the oxidesemiconductor layer 2003; and an electrode 2020 which is in contact withthe electrode 2005 b through an opening portion provided in theprotective insulating layer 2007.

Moreover, the display device includes, in the cross section B1-B2, anelectrode 2008 over the substrate 2000; the gate insulating layer 2002over the electrode 2008; the protective insulating layer 2007 providedover the gate insulating layer 2002; and the electrode 2020 providedover the protective insulating layer 2007.

Electrodes 2022 and 2029 and electrodes 2023, 2024, and 2028 serve as awiring or an electrode for connection with an FPC.

As the substrate 2000, a substrate which can be applied to the substrate210 in Embodiment 1 can be used.

The gate electrode 2001 and the electrodes 2008, 2022, and 2023 can beformed using a material and a method which can be applied to those ofthe gate electrodes 2111 and 2112 in Embodiment 1.

The gate insulating layer 2002 can be formed using a material and amethod which can be applied to those of the gate insulating layer 212 inEmbodiment 1. In this embodiment, a 50-nm-thick silicon oxide film isformed as the gate insulating layer 2002.

The oxide semiconductor layer 2003 can be formed using a material and amethod which can be applied to those of the oxide semiconductor layers2131 and 2132 in the above embodiments, for example. Here, the oxidesemiconductor layer 2003 is formed by depositing an In—Ga—Zn—O-basednon-single-crystal film using an 8-inch diameter oxide semiconductortarget containing In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1) in an argonatmosphere or an oxygen atmosphere under the following conditions: thedistance between the substrate and the target is 170 mm, the pressure is0.4 Pa, and the direct-current (DC) power supply is 0.5 kW. Note that itis preferable to use a pulsed direct-current (DC) power supply becausedust can be reduced and film thickness distribution is uniform. TheIn—Ga—Zn—O-based non-single-crystal film preferably has a thickness of 5nm to 200 nm. In this embodiment, the thickness of the In—Ga—Zn—O-basednon-single-crystal film is 100 nm. Moreover, reverse sputtering can beperformed before the oxide semiconductor film is formed.

The oxide semiconductor layers 2004 a and 2004 b can be formed using amaterial and a method which can be applied to those of the oxidesemiconductor layers 2141 a, 2141 b, 2142 a, and 2142 b in the aboveembodiments, for example. Here, the oxide semiconductor layers 2004 aand 2004 b are formed by depositing an In—Ga—Zn—O-basednon-single-crystal film using a target in which the composition ratio isIn₂O₃:Ga₂O₃:ZnO=1:1:1 by sputtering under the following depositionconditions: the pressure is 0.4 Pa, the power is 500 W, the depositiontemperature is room temperature, and the flow rate of an argon gas is 40sccm. Note that an In—Ga—Zn—O-based non-single-crystal film having acrystal grain of 1 nm to 10 nm just after deposition is sometimes formedin spite of intentionally using a target in which the composition ratiois In₂O₃:Ga₂O₃:ZnO=1:1:1. Further, by adjusting the ratio of componentsof the target, the pressure for deposition (0.1 Pa to 2.0 Pa), the power(250 W to 3000 W: 8 inches in diameter), the temperature (roomtemperature to 100° C.), a deposition condition for reactive sputtering,and the like as appropriate, the presence or existence of crystal grainsand the density of crystal grains can be adjusted and the diameter ofcrystal grains can be adjusted in the range of 1 nm to 10 nm. Thethickness of the In—Ga—Zn—O-based non-single-crystal film is preferably5 nm to 20 nm. It is needless to say that when crystal grains areincluded in the film, the size of the crystal grains is not greater thanthe thickness of the film. In this embodiment, the thickness of theoxide semiconductor layers 2004 a and 2004 b is 5 nm.

Note that the deposition conditions of the In—Ga—Zn—O-basednon-single-crystal film serving as the oxide semiconductor layer 2003are made different from those of the In—Ga—Zn—O-based non-single-crystalfilm serving as the oxide semiconductor layers 2004 a and 2004 b. Forexample, the ratio of the flow rate of an oxygen gas to the flow rate ofan argon gas in the deposition conditions of the In—Ga—Zn—O-basednon-single-crystal film serving as the oxide semiconductor layer 2003 ishigher than that in the deposition conditions of the In—Ga—Zn—O-basednon-single-crystal film serving as the oxide semiconductor layers 2004 aand 2004 b. Specifically, the In—Ga—Zn—O-based non-single-crystal filmserving as the oxide semiconductor layers 2004 a and 2004 b is depositedin a rare gas (e.g., argon or helium) atmosphere (or an atmosphere withoxygen gas of 10% or less and argon gas of 90% or more), and theIn—Ga—Zn—O-based non-single-crystal film serving as the oxidesemiconductor layer 2003 is deposited in an oxygen atmosphere (or anatmosphere in which the flow rate of an oxygen gas is equal to or morethan that of an argon gas).

The In—Ga—Zn—O-based non-single-crystal film serving as the oxidesemiconductor layers 2004 a and 2004 b may be deposited in a chamberwhich is the same as or different from that in which reverse sputteringhas been performed.

Among sputtering methods, there are an RF sputtering method using ahigh-frequency power supply as a sputtering power supply, a DCsputtering method, and also a pulsed DC sputtering method in whichpulsed bias is applied. The RF sputtering method is mainly used fordepositing an insulating film, and the DC sputtering method is mainlyused for depositing a metal film.

Moreover, there is a multi-source sputtering apparatus in which aplurality of targets of different materials can be arranged. With themulti-source sputtering apparatus, films of different materials can bestacked in one chamber, or plural kinds of materials can be deposited byelectric discharge at a time in one chamber.

Further, there are a sputtering apparatus which includes a magneticmechanism inside a chamber and employs a magnetron sputtering method;and a sputtering apparatus which employs an ECR sputtering method usingplasma generated by using a microwave without glow discharge.

Furthermore, as a deposition method using a sputtering method, there area reactive sputtering method in which a target substance and asputtering gas component chemically react with each other duringdeposition to form a thin film of a compound of these materials; and abias sputtering method in which voltage is also applied to a substrateduring deposition.

The electrodes 2005 a, 2005 b, and 2024 can be formed using a materialand a method which can be applied to those of the electrodes 215, 216,and 217 in the above embodiments, for example. Here, the electrodes 2005a, 2005 b, and 2024 have a single-layer structure of a titanium film.

In addition, oxygen plasma treatment may be performed on a channelregion of the oxide semiconductor layer 2003. By performing the oxygenplasma treatment, a TFT can be normally off. Moreover, by performing theplasma treatment, damage to the oxide semiconductor layer 2003 byetching can be repaired. The oxygen plasma treatment is preferablyperformed in an atmosphere of O₂ or N₂O, preferably an atmosphere of N₂,He, or Ar which contains oxygen. Alternatively, the oxygen plasmatreatment may be performed in an atmosphere where Cl₂ or CF₄ is added tothe above atmosphere.

As the protective insulating layer 2007, a silicon nitride film, asilicon oxide film, a silicon oxynitride film, an aluminum oxide film, atantalum oxide film, or the like which is obtained by a sputteringmethod or the like can be used. Note that when a non-reducible film(such as a silicon oxide film) is used as the protective insulatinglayer 2007, a channel region of a TFT subjected to the above oxygenplasma treatment is protected, and shift of the threshold voltage overtime can be suppressed.

The electrodes 2020, 2029, and 2028 are formed using indium oxide(In₂O₃), an alloy of indium oxide and tin oxide (In₂O₃—SnO₂, referred toas ITO), or the like by a sputtering method, a vacuum evaporationmethod, or the like. Such a material is etched with a hydrochloricacid-based solution. Note that since etching of ITO particularly tendsto leave residue, an alloy of indium oxide and zinc oxide (In₂O₃—ZnO)may be used in order to improve the etching processability.

FIGS. 23A and 23B are a cross-sectional view and a top view of a gatewiring terminal portion at this stage, respectively. FIG. 23A is across-sectional view along C1-C2 in FIG. 23B. In FIG. 23A, a transparentconductive film 2055 formed over a protective insulating film 2054 is aterminal electrode for connection, which functions as an input terminal.Further, in FIG. 23A, in the terminal portion, a first terminal 2051which is formed of the same material as a gate wiring and a connectionelectrode 2053 which is formed of the same material as a source wiringoverlap with each other with a gate insulating layer 2052 therebetweenand are in direct contact with each other to allow electricalcontinuity. Moreover, the connection electrode 2053 and the transparentconductive film 2055 are in direct contact with each other through acontact hole provided in the protective insulating film 2054 to allowelectrical continuity.

FIGS. 23C and 23D are a cross-sectional view and a top view of a sourcewiring terminal portion, respectively. FIG. 23C is a cross-sectionalview along D1-D2 in FIG. 23D. In FIG. 23C, the transparent conductivefilm 2055 formed over the protective insulating film 2054 is a terminalelectrode for connection, which functions as an input terminal.Moreover, in FIG. 23C, in the terminal portion, an electrode 2056 whichis formed of the same material as the gate wiring is placed below asecond terminal 2050 which is electrically connected to the sourcewiring, so as to overlap with the second terminal 2050 with the gateinsulating layer 2052 therebetween. The electrode 2056 is notelectrically connected to the second terminal 2050. When the electrode2056 is set to have a potential different from that of the secondterminal 2050, for example, a floating potential, GND, or 0 V,capacitance for preventing noise or static electricity can be formed.Further, the second terminal 2050 is electrically connected to thetransparent conductive film 2055 through the protective insulating film2054.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided based on the pixel density. Moreover, a plurality of firstterminals at the same potential as the gate wiring, second terminals atthe same potential as the source wiring, third terminals at the samepotential as the capacitor wiring, and the like are arranged in theterminal portion. The number of each of the terminals can be a givennumber and is determined as appropriate.

Accordingly, a pixel TFT portion including the TFT, which is abottom-gate n-channel TFT, and a storage capacitor can be completed.Then, they are arranged in matrix corresponding to pixels so that apixel portion is formed; thus, a substrate for manufacturing an activematrix display device can be formed. In this specification, such asubstrate is referred to as an active matrix substrate for convenience.

When an active matrix liquid crystal display device is formed, a liquidcrystal layer is provided between an active matrix substrate and acounter substrate provided with a counter electrode, and the activematrix substrate and the counter substrate are fixed. A common electrodewhich is electrically connected to the counter electrode provided on thecounter substrate is provided over the active matrix substrate, and afourth electrode which is electrically connected to the common electrodeis provided in a terminal portion. The fourth terminal is a terminal formaking the common electrode have a fixed potential, for example, GND or0 V.

The n-channel transistor obtained in this embodiment uses theIn—Ga—Zn—O-based non-single-crystal film for a channel formation regionand thus has favorable dynamic characteristics, whereby the abovedriving techniques can be used in combination.

Further, when a light-emitting display device is formed, in order to setone electrode (also referred to as a cathode) of an organiclight-emitting element to have a low power supply voltage, for example,GND or 0 V, a fourth terminal for making the cathode have the low powersupply voltage such as GND or 0 V is provided in a terminal portion.Moreover, when the light-emitting display device is formed, a powersupply line is provided in addition to a source wiring and a gatewiring. Accordingly, a fifth terminal electrically connected to thepower supply line is provided in the terminal portion.

A gate line driver circuit or a source line driver circuit isconstituted by TFTs using an oxide semiconductor, whereby manufacturingcosts are reduced. Moreover, a gate electrode of the TFT included in thedriver circuit is directly connected to a source wiring or a drainwiring so that the number of contact holes is reduced, whereby a displaydevice can be provided in which the area occupied by the driver circuitis reduced.

Therefore, according to this embodiment, a highly reliable displaydevice with high electric characteristics can be provided at low cost.

Note that this embodiment can be implemented in combination with otherembodiments as appropriate.

Embodiment 8

In this embodiment, a light-emitting display device will be described asan example of the display device shown in Embodiment 6. As an example, alight-emitting display device in which electroluminescence is used for alight-emitting element will be described in this embodiment.

Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter as an inorganic EL element.

In an organic EL element, by application of voltage to thelight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and there flows a current. Then, these carriers (the electronsand the holes) are recombined, so that the light-emitting organiccompound is set in an excited state. The light-emitting emits light whenit returns from the excited state to a ground state. Based on such amechanism, such a light-emitting element is referred to as acurrent-excitation light-emitting element.

Inorganic EL elements are classified according to the element structuresinto a dispersion inorganic EL elements and thin-film inorganic ELelements. A dispersion inorganic EL element includes a light-emittinglayer where particles of a light-emitting material are dispersed in abinder, and its light emission mechanism is donor-acceptor recombinationlight emission utilizing a donor level and an acceptor level. Athin-film inorganic EL element has a structure in which a light-emittinglayer is sandwiched between dielectric layers, which are furthersandwiched between electrodes, and its light emission mechanism islocalized light emission utilizing inner-shell electron transition ofmetal ions. Note that here, an organic EL element is described as alight-emitting element.

A circuit configuration of a pixel in a display device in thisembodiment will be described with reference to FIG. 24. FIG. 24 is acircuit diagram illustrating a circuit configuration of a pixel of thedisplay device in this embodiment.

As illustrated in FIG. 24, the pixel of the display device in thisembodiment includes a transistor 851, a storage capacitor 852, atransistor 853, and a light-emitting element 854.

A gate of the transistor 851 is electrically connected to a scan line855, and one of a source and a drain thereof is electrically connectedto a signal line 856. A high power supply voltage is applied to theother of the source and the drain of the transistor 851 through thestorage capacitor 852.

A gate of the transistor 853 is electrically connected to the other ofthe source and the drain of the transistor 851. The high power supplyvoltage is applied to one of a source and a drain of the transistor 853.

The light-emitting element 854 has a first terminal and a secondterminal. The first terminal is electrically connected to the other ofthe source and the drain of the transistor 853. A low power supplyvoltage is applied to the second terminal.

Next, operation of the pixel illustrated in FIG. 24 will be described.

An example of display operation of the pixel in the display device inthis embodiment is described.

First, a pixel to which data is written is selected. In the selectedpixel, the transistor 851 is turned on by a scan signal input from thescan line 855, and a video signal (also referred to as a data signal),which is a fixed voltage, is input from the signal line 856 to the gateof the transistor 853.

The transistor 853 is turned on or off by a voltage in response to thedata signal input to the gate. When the transistor 853 is on, a voltageapplied between the first terminal and the second terminal of thelight-emitting element 854 depends on a gate voltage of the transistor853 and the high power supply voltage. At this time, current flowsthrough the light-emitting element 854 depending on the voltage appliedbetween the first terminal and the second terminal, and thelight-emitting element 854 emits light with illuminance in response tothe amount of current flowing therethrough. Further, since the gatevoltage of the transistor 853 is held for a certain period by thestorage capacitor 852, the light-emitting element 854 maintains alight-emitting state for a certain period.

When the data signal input from the signal line 856 to the pixel isdigital, the pixel enters into a light-emitting state or anon-light-emitting state by switching on and off of the transistor 851.Accordingly, gradation can be expressed by an area ratio grayscalemethod or a time ratio grayscale method. An area ratio grayscale methodrefers to a driving method by which one pixel is divided into aplurality of subpixels and each of the subpixels with the structureillustrated in FIG. 24 is independently driven based on a data signal sothat gradation is expressed. Further, a time ratio grayscale methodrefers to a driving method by which a period during which a pixel is ina light-emitting state is controlled so that gradation is expressed.

Since the response speed of light-emitting elements is higher than thatof liquid crystal elements or the like, the light-emitting elements aresuitable for a time ratio grayscale method as compared to the liquidcrystal elements. Specifically, when display is performed by a time grayscale method, one frame period is divided into a plurality of subframeperiods. Then, in accordance with video signals, the light-emittingelement in the pixel is set in a light-emitting state or anon-light-emitting state in each subframe period. By dividing one frameperiod into a plurality of subframe periods, the total length of aperiod in which pixels actually emit light in one frame period can becontrolled with video signals, and gradation can be expressed.

Among driver circuits in the light-emitting display device, part of adriver circuit which can be constituted by n-channel TFTs can be formedover a substrate where TFTs in a pixel portion are formed. Moreover, asignal line driver circuit and a scan line driver circuit can beconstituted only by n-channel TFTs.

Next, a structure of a light-emitting element will be described withreference to FIGS. 25A to 25C. Here, a cross-sectional structure of apixel in the case of an n-channel driving TFT is described as anexample. TFTs 7001, 7011, and 7021, which are driving TFTs used in adisplay device in FIGS. 25A, 25B, and 25C respectively, can be formed ina similar manner to the TFTs shown in the above embodiments, include anoxide semiconductor layer as a semiconductor layer, and have highreliability.

In order to extract light emitted from a light-emitting element, atleast one of an anode and a cathode needs to be transparent. A TFT and alight-emitting element are formed over a substrate. There arelight-emitting elements having a top emission structure in which lightis extracted through the surface opposite to the substrate, having abottom emission structure in which light is extracted through thesurface on the substrate side, and having a dual emission structure inwhich light is extracted through the surface on the substrate side andthe surface opposite to the substrate. The pixel structure of thepresent invention can be applied to a light-emitting element having anyof these emission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 25A.

FIG. 25A is a cross-sectional view of a pixel in the case where the TFT7001, which is the driving TFT, is an n-channel TFT and light emittedfrom a light-emitting element 7002 passes through an anode 7005. In FIG.25A, a cathode 7003 of the light-emitting element 7002 and the TFT 7001,which is the driving TFT, are electrically connected to each other, anda light-emitting layer 7004 and the anode 7005 are sequentially stackedover the cathode 7003. As the cathode 7003, any conductive film can beused as long as it has a low work function and reflects light. Forexample, Ca, Al, CaF, MgAg, AlLi, or the like is preferably used. Thelight-emitting layer 7004 may be formed using a single layer or bystacking a plurality of layers. When the light-emitting layer 7004 isformed using a plurality of layers, the light-emitting layer 7004 isformed by stacking an electron-injecting layer, an electron-transportinglayer, a light-emitting layer, a hole-transporting layer, and ahole-injecting layer in order over the cathode 7003. Note that it is notnecessary to form all of these layers. The anode 7005 is formed using alight-transmitting conductive film such as a film of indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thecathode 7003 and the anode 7005 sandwich the light-emitting layer 7004.In the pixel illustrated in FIG. 25A, light is emitted from thelight-emitting element 7002 to the anode 7005 side as shown by an arrow.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 25B. FIG. 25B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is an n-channelTFT and light emitted from a light-emitting element 7012 passes througha cathode 7017. In FIG. 25B, the cathode 7017 of the light-emittingelement 7012 is formed over a light-transmitting conductive film 7013which is electrically connected to the driving TFT 7011, and alight-emitting layer 7014 and an anode 7015 are sequentially stackedover the cathode 7017. Note that when the anode 7015 has alight-transmitting property, a light-blocking film 7016 for reflectingor blocking light may be formed so as to cover the anode 7015. As in thecase of FIG. 25A, a variety of materials can be used for the cathode7017 as long as a material is a conductive material having a low workfunction. Note that the cathode 7017 has a thickness that can transmitlight (preferably has approximately 5 nm to 30 nm). For example, a20-nm-thick aluminum film can be used as the cathode 7017. Thelight-emitting layer 7014 may be formed of a single layer or by stackinga plurality of layers as in FIG. 25A. The anode 7015 is not necessary totransmit light, but can be formed using a light-transmitting conductivefilm as in FIG. 25A. The light-blocking film 7016 can be formed using,for example, a metal which reflects light; however, one embodiment ofthe invention is not limited to a metal film. For example, a resin towhich a black pigment is added can be used.

The light-emitting element 7012 corresponds to a region where thecathode 7017 and the anode 7015 sandwich the light-emitting layer 7014.In the pixel illustrated in FIG. 25B, light is emitted from thelight-emitting element 7012 to the cathode 7017 side as shown by anarrow.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 25C. In FIG. 25C, a cathode 7027 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7023 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are sequentiallystacked over the cathode 7027. As in the case of FIG. 25A, a variety ofmaterials can be used for the cathode 7027 as long as a material is aconductive material with a low work function. Note that the cathode 7027has a thickness that can transmit light. For example, Al having athickness of 20 nm can be used as the cathode 7027. As in FIG. 25A, thelight-emitting layer 7024 may be formed using a single layer or a stackof a plurality of layers. The anode 7025 can be formed using alight-transmitting conductive film as in FIG. 25A.

The light-emitting element 7022 corresponds to a region where thecathode 7027, the light-emitting layer 7024, and the anode 7025 overlapwith each other. In the pixel illustrated in FIG. 25C, light is emittedfrom the light-emitting element 7022 to both the anode 7025 side and thecathode 7027 side as shown by arrows.

Note that although an organic EL element is described here as alight-emitting element, an inorganic EL element can also be provided asa light-emitting element.

Note that in this embodiment, the example is described in which a TFT(also referred to as a driving TFT) which controls driving of alight-emitting element is electrically connected to the light-emittingelement; alternatively, a structure may be employed in which a TFT forcurrent control is connected between the driving TFT and thelight-emitting element.

Next, the appearance and cross section of the display device (alsoreferred to as a light-emitting panel) in this embodiment will bedescribed with reference to FIGS. 26A and 26B. FIG. 26A is a top view ofthe display device in this embodiment, in which a TFT and alight-emitting element formed over a first substrate are sealed betweenthe first substrate and a second substrate by a sealing material. FIG.26B is a cross-sectional view along H—I in FIG. 26A.

A sealing material 4505 is provided so as to surround a pixel portion4502, a signal line driver circuits 4503 a and 4503 b, and scan linedriver circuits 4504 a and 4504 b which are provided over a firstsubstrate 4501. Moreover, a second substrate 4506 is provided over thepixel portion 4502, the signal line driver circuits 4503 a and 4503 b,and the scan line driver circuits 4504 a and 4504 b. Accordingly, thepixel portion 4502, the signal line driver circuits 4503 a and 4503 b,and the scan line driver circuits 4504 a and 4504 b are sealed, togetherwith a filler 4507, with the first substrate 4501, the sealing material4505, and the second substrate 4506. In such a manner, it is preferableto pack (seal) the pixel portion 4502, the signal line driver circuits4503 a and 4503 b, and the scan line driver circuits 4504 a and 4504 bwith a protective film (such as an attachment film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the pixel portion 4502, the signal linedriver circuits 4503 a and 4503 b, and the scan line driver circuits4504 a and 4504 b are not exposed to the air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b, which are formedover the first substrate 4501, each include a plurality of TFTs. In FIG.26B, a TFT 4510 included in the pixel portion 4502 and a TFT 4509included in the signal line driver circuit 4503 a are illustrated as anexample.

As the TFTs 4509 and 4510, the highly reliable TFT shown in Embodiment4, which includes the oxide semiconductor layer as a semiconductorlayer, can be used. Alternatively, the TFT shown in Embodiment 5 may beused. In this embodiment, the TFTs 4509 and 4510 are n-channel TFTs.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode or a drain electrode of the TFT 4510. Note that thelight-emitting element 4511 has a layered structure of the firstelectrode 4517, an electroluminescent layer 4512, and a second electrode4513; however, the structure of the light-emitting element is notlimited to that shown in this embodiment. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

A bank 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. In particular, it ispreferable that the bank 4520 be formed using a photosensitive materialto have an opening portion over the first electrode 4517, and a sidewallof the opening portion be formed as an inclined surface with acontinuous curvature.

The electroluminescent layer 4512 may be formed using a single layer ora stack of a plurality of layers.

In order to prevent oxygen, hydrogen, moisture, carbon dioxide, or thelike from entering the light-emitting element 4511, a protective layermay be formed over the second electrode 4513 and the bank 4520. As theprotective layer, a silicon nitride film, a silicon nitride oxide film,a DLC (diamond like carbon) film, or the like can be formed.

Further, a variety of signals and potentials are supplied to the signalline driver circuits 4503 a and 4503 b, the scan line driver circuits4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 a and 4518b.

In this embodiment, a connection terminal electrode 4515 is formed usingthe same conductive film as the first electrode 4517 included in thelight-emitting element 4511. A terminal electrode 4516 is formed usingthe same conductive film as the source electrodes and the drainelectrodes of the TFTs 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used other than an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. In this embodiment, nitrogen is used for the filler 4507.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on a light-emitting surfaceof the light-emitting element. Further, a polarizing plate or acircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bedispersed on an uneven surface to reduce glare can be performed

As the signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b, a driver circuit formed using asingle crystal semiconductor film or a polycrystalline semiconductorfilm may be mounted on a substrate separately prepared. Alternatively,only the signal line driver circuit or part thereof, or the scan linedriver circuit or part thereof may be separately formed to be mounted.This embodiment is not limited to the structure in FIGS. 26A and 26B.

Through the above steps, a highly reliable light-emitting display device(display panel) can be manufactured.

Note that this embodiment can be combined with the structures disclosedin other embodiments as appropriate.

Embodiment 9

In this embodiment, electronic paper will be described as an example ofthe display device shown in Embodiment 6.

The logic circuit shown in the above embodiments can be used inelectronic paper. Electronic paper is also referred to as anelectrophoretic display device (an electrophoretic display) and hasadvantages of having high readability which is equivalent to normalpaper and lower power consumption than other display devices, and beingthin and lightweight.

A variety of modes of electrophoretic displays can be considered. Anelectrophoresis display includes a plurality of microcapsules whichinclude first particles having a positive charge and second particleshaving a negative charge, and are dispersed in a solvent or a solute. Byapplying an electrical field to the microcapsules, the particles in themicrocapsules move in opposite directions to each other, and only acolor of the particles gathered on one side is displayed. Note that thefirst particles or the second particles contain a dye and do not movewhen there is no electric field. Moreover, colors (including colorless)of the first particles and the second particles are different from eachother.

Accordingly, the electrophoretic display utilizes a so-calleddielectrophoretic effect, in which a substance with a high dielectricconstant moves to a region with high electric fields. Theelectrophoretic display does not require a polarizing plate and acounter substrate, which are necessary for a liquid crystal displaydevice, so that the thickness and weight of the electrophoretic displayare reduced by half.

A substance in which the microcapsules are dispersed in a solvent iscalled electronic ink, and the electronic ink can be printed on asurface of glass, plastic, fabric, paper, or the like. Moreover, colordisplay is possible with the use of a color filter or particlesincluding a coloring matter.

Further, when a plurality of the above microcapsules are arranged overan active matrix substrate so as to be placed between two electrodes, anactive matrix display device can be completed, and display can beperformed by application of electric fields to the microcapsules. Forexample, the active matrix substrate obtained with the TFT in Embodiment4 or Embodiment 5 can be used.

Note that for the first particles and the second particles in themicrocapsule, one or a composite material of a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, or a magnetophoretic material maybe used.

Next, an example of a structure of electronic paper in this embodimentwill be described with reference to FIG. 27. FIG. 27 is across-sectional view illustrating a structure of the electronic paper inthis embodiment.

The electronic paper illustrated in FIG. 27 includes a TFT 581 over asubstrate 580; insulating layers 583, 584, and 585 which are stackedover the TFT 581; an electrode 587 which is in contact with a sourceelectrode or a drain electrode of the TFT 581 through an opening portionprovided in the insulating layers 583 to 585; and includes between theelectrode 587 and an electrode 588 provided on a substrate 596,spherical particles 589, each of which includes a black region 590 a, awhite region 590 b, and a cavity 594 which surrounds the black region590 a and the white region 590 b and is filled with a liquid; and afiller 595 provided around the spherical particles 589.

The TFT 581 can be formed in a similar manner to the TFT shown inEmbodiment 4 and is a highly reliable TFT including an oxidesemiconductor layer as a semiconductor layer. Alternatively, the TFTshown in Embodiment 5 can be applied to the TFT 581 in this embodiment.

A method of using the spherical particles 589 is called a twisting balldisplay method. In the twisting ball display system, spherical particleseach colored in black and white are arranged between a first electrodeand a second electrode, which are electrodes used for a display element,and potential difference is generated between the first electrode andthe second electrode to control orientation of the spherical particles;accordingly, display is performed.

Further, instead of the spherical element, an electrophoretic elementcan also be used. A microcapsule having a diameter of approximately 10μm to 200 μM, in which a transparent liquid, positively charged whitemicroparticles, and negatively charged black microparticles areencapsulated, is used. In the microcapsule provided between the firstelectrode and the second electrode, when an electric field is applied bythe first electrode and the second electrode, the white microparticlesand the black microparticles move to opposite directions to each other,so that white or black can be displayed. An electrophoretic displayelement is a display element to which this principle is applied. Theelectrophoretic display element has higher reflectivity than a liquidcrystal display element, and thus, an assistant light is unnecessary.Moreover, power consumption is low, and a display portion can berecognized in a dusky place. Further, even when power is not supplied tothe display portion, an image which has been displayed once can bemaintained. Accordingly, a displayed image can be stored even if asemiconductor device having a display function (which may simply bereferred to as a display device or a semiconductor device provided witha display device) is distanced from an electric wave source.

The logic circuit which is one embodiment of the invention disclosed inthe specification can be used, for example, as a driver circuit for theelectronic paper in this embodiment. Further, since a thin filmtransistor using an oxide semiconductor layer can be applied to atransistor in the display portion, the driver circuit and the displayportion can be provided over one substrate, for example.

The electronic paper can be used in electronic devices of variousfields, which display information. For example, the electronic paper canbe applied to e-book readers (electronic books), posters, advertisementson vehicles such as trains, or displays on a variety of cards such ascredit cards. An example of such an electronic device will beillustrated in FIG. 28. FIG. 28 illustrates an example of an e-bookreader 2700.

As illustrated in FIG. 28, the e-book reader 2700 has two housings 2701and 2703. The housings 2701 and 2703 are united with an axis portion2711, and the e-book reader 2700 can be opened and closed with the axisportion 2711 as an axis. With such a structure, the e-book reader 2700can be operated like a paper book.

A display portion 2705 is incorporated into the housing 2701. A displayportion 2707 is incorporated into the housing 2703. The display portions2705 and 2707 may display one image or different images. When thedisplay portions display different images, text can be displayed on theright display portion (the display portion 2705 in FIG. 28) and an imagecan be displayed on the left display portion (the display portion 2707in FIG. 28), for example.

Further, FIG. 28 illustrates an example where the housing 2701 isprovided with an operation portion and the like. For example, thehousing 2701 is provided with a power supply switch 2721, operation keys2723, a speaker 2725, and the like. Pages can be turned by the operationkey 2723. Note that a keyboard, a pointing device, or the like may beprovided on the same side as the display portion in the housing.Moreover, a terminal for external connection (e.g., an earphoneterminal, a USB terminal, and a terminal capable of connecting a varietyof cables such as an AC adapter and a USB cable), a portion forinserting recording media, or the like may be provided on a rear surfaceor a side surface of the housing. Furthermore, the e-book reader 2700may functions as an electronic dictionary.

In addition, the e-book reader 2700 may wirelessly transmit and receiveinformation. The e-book reader 2700 can have a structure where desiredbook data or the like is wirelessly purchased and downloaded from ane-book server.

Embodiment 10

In this embodiment, a system-on-panel display device will be describedas one embodiment of the display device in Embodiment 6.

The logic circuit which is one embodiment of the invention disclosed inthis specification can be applied to a system-on-panel display device inwhich a display portion and a driver circuit are provided over onesubstrate. A specific structure of the display device will be describedbelow.

The display device in this embodiment includes a display element. As thedisplay element, a liquid crystal element (also referred to as a liquidcrystal display element) or a light-emitting element (also referred toas a light-emitting display element) can be used. A light-emittingelement includes, in its category, an element whose luminance iscontrolled by current or voltage, and specifically an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

In addition, the display device in this embodiment includes, in itscategory, a panel in which a display element is sealed, and a module inwhich an IC and the like including a controller are mounted on thepanel. Moreover, this embodiment relates to an element substrate beforea display element is completed in a process of manufacturing the displaydevice. The element substrate is provided with a means for supplyingcurrent to the display element in each of a plurality of pixels.Specifically, the element substrate may be in a state where only a pixelelectrode of the display element is provided, a state after a conductivefilm to serve as a pixel electrode is formed and before the conductivefilm is etched to form the pixel electrode, or other states.

Note that a display device in this specification refers to an imagedisplay device, a display device, or a light source (including alighting device). Further, the display device includes any of thefollowing modules in its category: a module including a connector suchas a flexible printed circuit (FPC), tape automated bonding (TAB) tape,or tape carrier package (TCP); a module including TAB tape or TCP whichis provided with a printed wiring board at the end thereof; and a moduleincluding an integrated circuit (IC) which is directly mounted on adisplay element by a chip on glass (COG) method.

Next, the appearance and cross section of a liquid crystal display panelwhich is one embodiment of the display device in this embodiment will bedescribed with reference to FIGS. 29A to 29C.

Each of FIGS. 29A and 29B is a top view of the display device in thisembodiment, in which a liquid crystal element 4013 and TFTs 4010 and4011 including the In—Ga—Zn—O-based non-single-crystal film shown inEmbodiment 4, which is formed over a first substrate 4001, as asemiconductor layer are sealed between the first substrate 4001 and asecond substrate 4006 with a sealing material 4005. FIG. 29C is across-sectional view along M-N in FIGS. 29A and 29B.

In the display panel in this embodiment, the sealing material 4005 isprovided so as to surround a pixel portion 4002 and a scan line drivercircuit 4004 which are provided over the first substrate 4001. Thesecond substrate 4006 is provided over the pixel portion 4002 and thescan line driver circuit 4004. Accordingly, the pixel portion 4002 andthe scan line driver circuit 4004 as well as a liquid crystal layer 4008are sealed with the first substrate 4001, the sealing material 4005, andthe second substrate 4006. Moreover, a signal line driver circuit 4003,which is formed using a single crystal semiconductor film or apolycrystalline semiconductor film over a substrate prepared separately,is provided in a region different from the region surrounded by thesealing material 4005 over the first substrate 4001.

Note that there is no particular limitation on a connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 29Aillustrates an example where the signal line driver circuit 4003 ismounted by a COG method. FIG. 29B illustrates an example where thesignal line driver circuit 4003 is mounted by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004, which areprovided over the first substrate 4001, each include a plurality ofTFTs. FIG. 29C illustrates the TFT 4010 included in the pixel portion4002 and the TFT 4011 included in the scan line driver circuit 4004.Insulating layers 4020 and 4021 are provided over the TFTs 4010 and4011.

As the TFTs 4010 and 4011, the highly reliable TFT shown in Embodiment4, which includes the oxide semiconductor layer as a semiconductorlayer, can be used. Alternatively, the TFT shown in Embodiment 5 may beused. In this embodiment, the TFTs 4010 and 4011 are n-channel TFTs.

A pixel electrode 4030 included in the liquid crystal element 4013 iselectrically connected to the TFT 4010. A counter electrode 4031 of theliquid crystal element 4013 is formed on the second substrate 4006. Theliquid crystal element 4013 corresponds to a region where the pixelelectrode 4030, the counter electrode 4031, and the liquid crystal layer4008 overlap with each other. The pixel electrode 4030 and the counterelectrode 4031 are provided with insulating layers 4032 and 4033functioning as alignment films, respectively, and sandwich the liquidcrystal layer 4008 with the insulating layers 4032 and 4033therebetween.

To the first substrate 4001 and the second substrate 4006, a materialand a manufacturing method which can be applied to those of thesubstrate 210 in the above embodiments can be applied.

A spacer 4035 is a columnar partition obtained by selective etching ofan insulating film, and is provided in order to control a distance (acell gap) between the pixel electrode 4030 and the counter electrode4031. Note that a spherical spacer may be used. Further, the counterelectrode 4031 is electrically connected to a common potential lineprovided over the same substrate as the TFT 4010. The counter electrode4031 and the common potential line can be electrically connected to eachother through conductive particles arranged between the pair ofsubstrates. Note that the conductive particles are included in thesealing material 4005.

Note that although this embodiment shows an example of a transmissiveliquid crystal display device, the present invention can also be appliedto a reflective liquid crystal display device or a transflective liquidcrystal display device.

As the liquid crystal display device in this embodiment, an example isshown in which a polarizing plate is provided on the outer side of thesubstrate (on the viewer side) and a color layer and the electrode usedfor the display element are sequentially provided on the inner side;alternatively, a polarizing plate may be provided on the inner side ofthe substrate. Moreover, a layered structure of the polarizing plate andthe color layer is not limited to that in this embodiment, and may bedetermined as appropriate depending on materials of the polarizing plateand the color layer or the conditions of the manufacturing process.Further, a light-blocking film functioning as a black matrix may beprovided.

In this embodiment, in order to reduce surface unevenness of the TFT andimprove the reliability of the TFT, the TFT is covered with aninsulating layer (the insulating layers 4020 and 4021) functioning as aprotective layer or a planarization insulating film. Note that theprotective layer prevents penetration of contaminating impurities suchas an organic matter, metal, or moisture included in the air, and thusis preferably dense. The protective layer may be formed by a sputteringmethod with a single layer or a stack of a silicon oxide film, a siliconnitride film, a silicon oxynitride film, a silicon nitride oxide film,an aluminum oxide film, an aluminum nitride film, an aluminum oxynitridefilm, or an aluminum nitride oxide film. The example where theprotective layer is formed by a sputtering method is shown in thisembodiment; however, one embodiment of the invention is not particularlylimited thereto, and the protective layer may be formed by a variety ofmethods. Further, by using the non-reducible film, the protective layercan also function as a reduction prevention layer.

Here, the insulating layer 4020 with a layered structure is formed asthe protective layer. In this case, as a first layer of the insulatinglayer 4020, a silicon oxide film is formed by a sputtering method. Theuse of the silicon oxide film as the protective layer is effective inpreventing hillocks in an aluminum film used as a source electrode and adrain electrode.

Moreover, an insulating layer is formed as a second layer of theprotective layer. Here, as the second layer of the insulating layer4020, a silicon nitride film is formed by a sputtering method. The useof the silicon nitride film as the protective layer can prevent mobileions such as sodium from entering the semiconductor region and changingelectric characteristics of the TFT.

Further, after the protective layer is formed, annealing (250° C. to400° C.) may be performed on the semiconductor layer.

Then, the insulating layer 4021 is formed as a planarization insulatingfilm. An organic material having heat resistance, such as polyimide,acrylic, polyimideamide, benzocyclobutene, polyamide, or epoxy can beused for the insulating layer 4021. Other than such organic materials,it is also possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like. Note that the insulatinglayer 4021 may be formed by stacking a plurality of insulating filmsformed of such materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having a Si—O—Si bond. Thesiloxane-based resin may include an organic group (e.g., an alkyl groupor an aryl group) or a fluoro group as a substituent. The organic groupmay include a fluoro group.

There is no particular limitation on the method of forming theinsulating layer 4021, and the insulating layer 4021 can be formed byany of the following methods and means depending on its material: asputtering method, an SOG method, spin coating, dip coating, spraycoating, a droplet discharging method (e.g., an ink jet method, screenprinting, or offset printing), a doctor knife, a roll coater, a curtaincoater, a knife coater, and the like. When the insulating layer 4021 isformed using a material liquid, the semiconductor layer may be annealed(300° C. to 400° C.) in a step of baking the insulating layer 4021. Thestep of baking the insulating layer 4021 serves to anneal thesemiconductor layer, whereby the display device can be efficientlymanufactured.

The pixel electrode 4030 and the counter electrode 4031 can be formedusing a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

Alternatively, the pixel electrode 4030 and the counter electrode 4031can be formed using a conductive composition containing a conductivemacromolecule (also referred to as a conductive polymer). The electrodeformed using a conductive composition preferably has a sheet resistanceof 10000 ohms/square or less and a light transmittance of 70% or more ata wavelength of 550 nm. Moreover, the resistivity of the conductivepolymer contained in the conductive composition is preferably equal toor less than 0.1 Ω·cm.

As the conductive polymer, a so-called π-electron conjugated conductivepolymer can be used. For example, polyaniline or a derivative thereof,polypyrrole or a derivative thereof, polythiophene or a derivativethereof, a copolymer of more than two kinds of these materials, and thelike can be given.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003, which is formed separately, the scan linedriver circuit 4004, and the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed usingthe same conductive film as the pixel electrode 4030 included in theliquid crystal element 4013. A terminal electrode 4016 is formed usingthe same conductive film as the source electrodes and the drainelectrodes of the TFTs 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal of the FPC 4018 through an anisotropic conductive film 4019.

Note that FIGS. 29A to 29C illustrate the example in which the signalline driver circuit 4003 is separately formed and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

As described above, a system-on-panel display device can be formed. Forthe display device in this embodiment, the logic circuit in the aboveembodiments can be used in the driver circuit, for example, and thelogic circuit can be formed in the same process as the TFT in thedisplay portion.

Note that this embodiment can be combined with the structures disclosedin other embodiments as appropriate.

Embodiment 11

The display devices shown in Embodiments 6 to 10 can be applied to avariety of electronic devices (including amusement machines). Examplesof electronic devices are television devices (also referred to astelevisions or television receivers), monitors for computers and thelike, cameras such as digital cameras and digital video cameras, digitalphoto frames, mobile phone devices (also referred to as mobile phones orcellular phones), portable game machines, portable informationterminals, sound reproducing devices, and large game machines such aspachinko machines.

FIG. 30A illustrates an example of a television device 9600. In thetelevision device 9600, a display portion 9603 is incorporated into ahousing 9601. The display portion 9603 can display an image. Further,the housing 9601 is supported by a stand 9605 here.

The television device 9600 can be operated with an operation switch ofthe housing 9601 or a separate remote controller 9610. Channels andvolume can be controlled with an operation key 9609 of the remotecontroller 9610 so that an image displayed on the display portion 9603can be controlled. Further, the remote controller 9610 may be providedwith a display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television device 9600 is provided with a receiver, amodem, and the like. With the receiver, general television broadcast canbe received. Further, when the television device 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers) data communication canbe performed.

FIG. 30B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated into a housing 9701. The display portion 9703 can display avariety of images. For example, the display portion 9703 can displaydata of an image taken with a digital camera or the like and function asa normal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (e.g., a USB terminal, or aterminal which can be connected to various cables such as a USB cable),a recording medium insertion portion, and the like. Although thesecomponents may be provided on the surface on which the display portionis provided, it is preferable to provide them on the side surface or therear surface for the design of the digital photo frame 9700. Forexample, a memory storing data of an image taken with a digital camerais inserted in the recording medium insertion portion of the digitalphoto frame, and the image data can be transferred and then displayed onthe display portion 9703.

Further, the digital photo frame 9700 may be configured to transmit andreceive data wirelessly. The structure may be employed in which desiredimage data is transferred wirelessly to be displayed.

FIG. 31A is a portable game machine and includes two housings of ahousing 9881 and a housing 9891, which are connected with a jointportion 9893 so that the portable game machine can be opened and folded.A display portion 9882 is incorporated into the housing 9881, and adisplay portion 9883 is incorporated into the housing 9891. Moreover,the portable game machine illustrated in FIG. 31A is provided with aspeaker portion 9884, a recording medium insertion portion 9886, an LEDlamp 9890, input means (operation keys 9885, a connection terminal 9887,a sensor 9888 (having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotation number,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radial ray, flow rate, humidity, gradient, vibration, odor, or infraredray), and a microphone 9889), and the like. It is needless to say thatthe structure of the portable game machine is not limited to thatdescribed above. The portable game machine may have a structure in whichadditional accessory equipment is provided as appropriate as long as atleast a display device is provided. The portable game machine in FIG.31A has a function of reading a program or data stored in a recordingmedium to display it on the display portion, and a function of sharinginformation with another portable game machine by wirelesscommunication. Note that a function of the portable game machine in FIG.31A is not limited to those described above, and the portable gamemachine can have a variety of functions.

FIG. 31B illustrates an example of a slot machine 9900, which is a largeamusement machine. In the slot machine 9900, a display portion 9903 isincorporated into a housing 9901. Moreover, the slot machine 9900 isprovided with operation means such as a start lever and a stop switch, acoin slot, a speaker, and the like. Needless to say, the structure ofthe slot machine 9900 is not limited to the above structure. The slotmachine may have a structure in which additional accessory equipment isprovided as appropriate as long as at least the display device accordingto the present invention is provided.

FIG. 32A illustrates an example of a mobile phone 9000. The mobile phone9000 is provided with a display portion 9002 incorporated into a housing9001, an operation button 9003, an external connection port 9004, aspeaker 9005, a microphone 9006, and the like.

When the display portion 9002 of the mobile phone 9000 illustrated inFIG. 32A is touched with a finger or the like, data can be input intothe mobile phone 9000. Further, operation such as making calls andtexting can be performed by touching the display portion 9002 with afinger or the like.

There are mainly three screen modes of the display portion 9002. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode, which is a combination of the twomodes, that is, a combination of the display mode and the input mode.

For example, in the case of making a call or texting, a text input modemainly for inputting text is selected for the display portion 9002 sothat characters displayed on a screen can be input. In that case, it ispreferable to display a keyboard or number buttons on almost all area ofthe screen of the display portion 9002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 9000, display on the screen of the display portion 9002 canbe automatically changed by determining the orientation of the mobilephone 9000 (whether the mobile phone 9000 stands upright or is laid downon its side).

The screen modes are changed by touching the display portion 9002 orusing the operation buttons 9003 of the housing 9001. Alternatively, thescreen modes may be changed depending on the kind of the image displayedon the display portion 9002. For example, when a signal of an imagedisplayed on the display portion is data of moving images, the screenmode is changed to the display mode. When the signal is text data, thescreen mode is changed to the input mode.

Further, in the input mode, when input by touching the display portion9002 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 9002 is detected, the screen modemay be controlled so as to be changed from the input mode to the displaymode.

The display portion 9002 can also function as an image sensor. Forexample, an image of a palm print, a fingerprint, or the like is takenwhen the display portion 9002 is touched with a palm or a finger,whereby personal identification can be performed. Further, when abacklight or a sensing light source which emits near-infrared light isprovided in the display portion, an image of a finger vein, a palm vein,or the like can be taken.

FIG. 32B illustrates another example of a mobile phone. The mobile phonein FIG. 32B includes a display device 9410 in a housing 9411, whichincludes a display portion 9412 and operation buttons 9413; and acommunication device 9400 in a housing 9401, which includes scan buttons9402, an external input terminal 9403, a microphone 9404, a speaker9405, and a light-emitting portion 9406 that emits light when receivinga call. The display device 9410 having a display function can bedetached from and attached to the communication device 9400 having atelephone function in two directions shown by arrows. Accordingly, shortaxes of the display device 9410 and the communication device 9400 can beattached to each other, or long axes of the display device 9410 and thecommunication device 9400 can be attached to each other. Further, whenonly a display function is necessary, the display device 9410 may bedetached from the communication device 9400 so that the semiconductordevice 9410 can be used by itself. The communication device 9400 and thedisplay device 9410 can transmit and receive images or input informationto/from each other by wireless communication or wired communication, andeach of the communication device 9400 and the display device 9410 has arechargeable battery.

Note that this embodiment can be combined with the structures disclosedin other embodiments as appropriate.

Example 1

In this example, an enhancement thin film transistor using an oxidesemiconductor will be described, in which oxygen plasma treatment isperformed as an example of oxygen vacancy control processing in order toshift the threshold voltage.

FIG. 33A illustrates a structure of a thin film transistor in thisexample.

The thin film transistor illustrated in FIG. 33A includes a gateelectrode 5002 over a substrate 5001, a gate insulating layer 5003 overthe gate electrode 5002, an oxide semiconductor layer 5004 over the gateinsulating layer 5003, and electrodes 5005 a and 5005 b to serve as asource electrode and a drain electrode.

In this example, a 100-nm-thick tungsten film was formed as the gateelectrode 5002; a 100-nm-thick SiON film as the gate insulating layer5003; a 50-nm-thick In—Ga—Zn—O-based non-single-crystal film as theoxide semiconductor layer 5004; and a 100-nm-thick titanium film as theelectrodes 5005 a and 5005 b.

Further, in this example, the threshold voltage of the thin filmtransistor is shifted by performing oxygen plasma treatment on a surfaceof a channel portion. The oxygen plasma treatment at this time wasperformed under the following conditions: the pressure in the chamberwas 0.4 P, the flow rate of an argon gas and oxygen was 10 sccm and 15sccm respectively, and the RF power was 500 W so that oxygen was madeinto plasma. In this example, the plasma treatment was performed for 5minutes.

FIG. 33B shows the result of measuring ID-VG of the transistor in thisexample before and after the oxygen plasma treatment.

As illustrated in FIG. 33B, the transistor before the oxygen plasmatreatment has a negative threshold voltage and is normally on as shownby a curve 5006, whereas the transistor after the oxygen plasmatreatment has a positive threshold voltage and is normally off as shownby a curve 5007. Accordingly, when oxygen plasma treatment is performedon a thin film transistor including an oxide semiconductor, thethreshold voltage of the transistor is shifted to a positive value andthe transistor serves as an enhancement transistor.

This application is based on Japanese Patent Application serial no.2008-281647 filed with Japan Patent Office on Oct. 31, 2008, the entirecontents of which are hereby incorporated by reference.

1. A logic circuit comprising: a first transistor having a gate, asource, and a drain, a second transistor having a gate, a source, and adrain, a first terminal electrically connected to the gate of the secondtransistor; and a second terminal electrically connected to a portionwhere the second transistor is connected to the first transistor,wherein a high power supply voltage terminal is electrically connectedto one of the source and the drain of the first transistor, and the gateof the first transistor is electrically connected to the other of thesource and the drain of the first transistor; wherein one of the sourceand the drain of the second transistor is electrically connected to theother of the source and the drain of the first transistor, and a lowpower supply voltage terminal is electrically connected to the other ofthe source and the drain of the second transistor, wherein each of thefirst transistor and the second transistor includes: a gate electrode; agate insulating layer provided over the gate electrode; a first oxidesemiconductor layer provided over the gate insulating layer; a sourceregion and a drain region in contact with part of the first oxidesemiconductor layer, wherein the source region and the drain region aresecond oxide semiconductor layers; a source electrode in contact withthe source region; and a drain electrode in contact with the drainregion, wherein the second transistor includes a reduction preventionlayer over the first oxide semiconductor layer, the source electrode,and the drain electrode, and wherein the first transistor does notinclude a reduction prevention layer over the first oxide semiconductorlayer, the source electrode, and the drain electrode.
 2. The logiccircuit according to claim 1, wherein the second transistor includes anoxygen vacancy control region between the source electrode and the drainelectrode over a surface of the first oxide semiconductor layer, whichis opposite to a surface in contact with the gate insulating layer. 3.The logic circuit according to claim 1, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layers containsindium, gallium, and zinc.
 4. The logic circuit according to claim 1,wherein the first transistor and the second transistor have the sameconductivity type.
 5. The logic circuit according to claim 1, whereinone of the source electrode and the drain electrode of the secondtransistor is in contact with the gate electrode of the first transistorthrough an opening portion provided in the gate insulating layer.
 6. Alogic circuit comprising: a first transistor having a gate, a source,and a drain, wherein a first clock signal is input to the gate of thefirst transistor, and an input signal is input to the one of the sourceand the drain of the first transistor; a first inverter having an inputterminal and an output terminal, the input terminal of the firstinverter electrically connected to the other of the source and the drainof the first transistor; a second inverter having an input terminal andan output terminal, the input terminal of the second inverterelectrically connected to the output terminal of the first inverter; athird inverter having an input terminal electrically connected to theoutput terminal of the first inverter, and an output terminal outputtingan output signal; and a second transistor having a gate, a source, and adrain, wherein a second clock signal is input to the gate of the secondtransistor, one of the source and the drain of the second transistor iselectrically connected to the other of the source and the drain of thefirst transistor, and the other of the source and the drain of thesecond transistor is electrically connected to the output terminal ofthe second inverter, wherein each of the first inverter and the secondinverter includes: a third transistor having a gate, a source, and adrain, a fourth transistor having a gate, a source, and a drain, a firstterminal electrically connected to the gate of the fourth transistor; asecond terminal electrically connected to a portion where the fourthtransistor is connected to the third transistor, wherein a high powersupply voltage terminal is electrically connected to one of the sourceand the drain of the third transistor, and the gate of the thirdtransistor is electrically connected to the other of the source and thedrain of the third transistor; wherein one of the source and the drainof the fourth transistor is electrically connected to the other of thesource and the drain of the third transistor, and a low power supplyvoltage terminal is electrically connected to the other of the sourceand the drain of the fourth transistor, wherein each of the thirdtransistor and the fourth transistor includes: a gate electrode; a gateinsulating layer provided over the gate electrode; a first oxidesemiconductor layer provided over the gate insulating layer; a sourceregion and a drain region in contact with part of the first oxidesemiconductor layer, wherein the source region and the drain region aresecond oxide semiconductor layers; a source electrode in contact withthe source region; and a drain electrode in contact with the drainregion, wherein the fourth transistor includes a reduction preventionlayer over the first oxide semiconductor layer, the source electrode,and the drain electrode, and wherein the third transistor does notinclude a reduction prevention layer over the first oxide semiconductorlayer, the source electrode, and the drain electrode.
 7. The logiccircuit according to claim 6, wherein the fourth transistor includes anoxygen vacancy control region between the source electrode and the drainelectrode over a surface of the first oxide semiconductor layer, whichis opposite to a surface in contact with the gate insulating layer. 8.The logic circuit according to claim 6, wherein each of the first oxidesemiconductor layer and the second oxide semiconductor layers containsindium, gallium, and zinc.
 9. The logic circuit according to claim 6,wherein the third transistor and the fourth transistor have the sameconductivity type.
 10. The logic circuit according to claim 6, whereinone of the source electrode and the drain electrode of the fourthtransistor is in contact with the gate electrode of the third transistorthrough an opening portion provided in the gate insulating layer.
 11. Alogic circuit comprising: a first transistor having a gate, a source,and a drain, a second transistor having a gate, a source, and a drain, afirst terminal electrically connected to the gate of the secondtransistor; a second terminal electrically connected to a portion wherethe second transistor is connected to the first transistor, wherein ahigh power supply voltage terminal is electrically connected to one ofthe source and the drain of the first transistor, and the gate of thefirst transistor is electrically connected to the other of the sourceand the drain of the first transistor; wherein one of the source and thedrain of the second transistor is electrically connected to the other ofthe source and the drain of the first transistor, and a low power supplyvoltage terminal is electrically connected to the other of the sourceand the drain of the second transistor, wherein each of the firsttransistor and the second transistor includes: a gate electrode; a gateinsulating layer provided over the gate electrode; an oxidesemiconductor layer provided over the gate insulating layer; and asource electrode and a drain electrode in contact with part of the oxidesemiconductor layer, wherein the second transistor includes a reductionprevention layer over the oxide semiconductor layer, the sourceelectrode, and the drain electrode, and wherein the first transistordoes not include a reduction prevention layer over the oxidesemiconductor layer, the source electrode, and the drain electrode. 12.The logic circuit according to claim 11, wherein the second transistorincludes an oxygen vacancy control region between the source electrodeand the drain electrode over a surface of the oxide semiconductor layer,which is opposite to a surface in contact with the gate insulatinglayer.
 13. The logic circuit according to claim 11, wherein the oxidesemiconductor layer contains indium, gallium, and zinc.
 14. The logiccircuit according to claim 11, wherein the first transistor and thesecond transistor have the same conductivity type.
 15. The logic circuitaccording to claim 11, wherein one of the source electrode and the drainelectrode of the second transistor is in contact with the gate electrodeof the first transistor through an opening portion provided in the gateinsulating layer.
 16. A logic circuit comprising: a first transistorhaving a gate, a source, and a drain, wherein a first clock signal isinput to the gate of the first transistor, and an input signal is inputto the one of the source and the drain of the first transistor; a firstinverter having an input terminal and an output terminal, the inputterminal of the first inverter electrically connected to the other ofthe source and the drain of the first transistor; a second inverterhaving an input terminal and an output terminal, the input terminal ofthe second inverter electrically connected to the output terminal of thefirst inverter; a third inverter having an input terminal electricallyconnected to the output terminal of the first inverter, and an outputterminal outputting an output signal; and a fourth transistor having agate, a source, and a drain, wherein a second clock signal is input tothe gate of the second transistor, one of the source and the drain ofthe second transistor is electrically connected to the other of thesource and the drain of the first transistor, and the other of thesource and the drain of the second transistor is electrically connectedto the output terminal of the second inverter, wherein each of the firstinverter and the second inverter includes: a third transistor having agate, a source, and a drain, a fourth transistor having a gate, asource, and a drain, a first terminal electrically connected to the gateof the fourth transistor; a second terminal electrically connected to aportion where the fourth transistor is connected to the thirdtransistor, wherein a high power supply voltage terminal is electricallyconnected to one of the source and the drain of the third transistor,and the gate of the third transistor is electrically connected to theother of the source and the drain of the third transistor; wherein oneof the source and the drain of the fourth transistor is electricallyconnected to the other of the source and the drain of the thirdtransistor, and a low power supply voltage terminal is electricallyconnected to the other of the source and the drain of the fourthtransistor; wherein each of the third transistor and the fourthtransistor includes: a gate electrode; a gate insulating layer providedover the gate electrode; an oxide semiconductor layer provided over thegate insulating layer; and a source electrode and a drain electrode incontact with part of the oxide semiconductor layer, wherein the fourthtransistor includes a reduction prevention layer over the oxidesemiconductor layer, the source electrode, and the drain electrode, andwherein the third transistor does not include a reduction preventionlayer over the oxide semiconductor layer, the source electrode, and thedrain electrode.
 17. The logic circuit according to claim 16, whereinthe fourth transistor includes an oxygen vacancy control region betweenthe source electrode and the drain electrode over a surface of the oxidesemiconductor layer, which is opposite to a surface in contact with thegate insulating layer.
 18. The logic circuit according to claim 16,wherein the oxide semiconductor layer contains indium, gallium, andzinc.
 19. The logic circuit according to claim 16, wherein the thirdtransistor and the fourth transistor have the same conductivity type.20. The logic circuit according to claim 16, wherein one of the sourceelectrode and the drain electrode of the fourth transistor is in contactwith the gate electrode of the third transistor through an openingportion provided in the gate insulating layer.